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Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
179
V ertebrates originated in theocean and have thrived thereever since. Roughly 350 million
years ago, vertebrates invaded the land as
well, an event that changed life on earth
forever. Descended from bony fishes,
land vertebrates had to adapt to the
harsher conditions ashore. They lost the
structural support that water provides and
had to develop ways of crawling or walk-
ing to get around. They evolved two pairs
of limbs. Because of this, land-dwelling
vertebrates—even snakes—are called
tetrapods, meaning “four-footed.”
Living on land also means having to
breathe air. Tetrapods evolved lungs,
which are internal air sacs that allow ab-
sorption of oxygen directly from air.
Tetrapods also had to evolve ways to keep
from drying out. The delicate egg is espe-
cially vulnerable, and the first land
tetrapods, the amphibians (class Am-
phibia), never really solved this problem.
Represented today by frogs, salamanders,
and their relatives, amphibians must keep
themselves moist, and most lay their eggs
in water. None of them are strictly marine.
Other groups of tetrapods solved the
problem of water loss and truly adapted 
to life on land. Reptiles (class Reptilia;
Fig. 9.1) evolved from now-extinct am-
phibians and for a long time were the
dominant land vertebrates The birds
(class Aves) and mammals (class Mam-
malia) both evolved from different groups
of now-extinct reptiles.
Humpback whale (Megaptera novaeangliae): mother with calf “under
wing,” Maui, Hawai‘i.
Marine Reptiles, Birds,
and Mammals
c h a p t e r
9
Domain
Bacteria DomainArchaea
Kingdom
Plantae KingdomAnimalia
Kingdom
Protista
Kingdom
Fungi
Domain
Eukaryota
Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
Having adapted to the land, various
groups of reptiles, birds, and mammals
turned around and reinvaded the ocean.
This chapter deals with these marine
tetrapods. Some, like sea turtles, have
not fully made the transition and still re-
turn to land to lay their eggs. Others,
like the humpback whales shown on
page 179, spend their entire lives at sea.
They have adapted so completely to a
marine existence that their streamlined
bodies look almost fish-like. This fish-
like appearance, however, belies the fact
that they evolved from animals that once,
about 55 million years ago, walked on
land (see “The Whales That Walked to
Sea,” p. 192). Their embryos even have
the four limbs that characterize all land
vertebrates (see Fig. 9.16).
The marine animals to be covered in
this chapter include some of the most
180 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
Domain
Bacteria
Domain
Archaea
Kingdom
Plantae
Kingdom
Fungi
Kingdom
Animalia
Kingdom
Protista
Order Pinnipedia
Seals, sea lions,
fur seals, walrus
Order Sphenisciformes
Penguins
Order Chelonia
Sea turtles
Order Squamata
Sea snakes,
marine iguana
Order Crocodilia
Saltwater
crocodile
Order Porcellariiformes
Tubenoses
Order Anseriformes
Ducks
Order Charadriiformes
Gulls and allies,
shorebirds
Order Ciconiiformes
Herons
Order Gruiformes
Rails, coots
Order Pelecaniformes
Pelicans and allies
Order Gaviiformes
Loons
Order Podicipediformes
Grebes
Order Carnivora
Sea otter, polar bear
Order Sirenia
Manatees, dugong
Order Cetacea
 Suborder Mysticeti
Baleen whales
Toothed whales
Suborder Odontoceti
CLASS MAMMALIA
Mammals
CLASS AVES
Birds
CLASS REPTILIA
Reptiles
Prokaryotes
Eukaryotes
Domain
Eukaryota
FIGURE 9.1 Classification scheme for marine reptiles, birds, and mammals.
Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
fascinating and awesome creatures on the
planet. Unfortunately, many are in dan-
ger of disappearing forever because of our
own greed. Some already have become
extinct.
MARINE REPTILES
There are around 7,000 living species 
of reptiles, including lizards, snakes, tur-
tles, and crocodiles. Their dry skin is cov-
ered with scales to prevent water loss.
Their eggs have a leathery shell that pre-
vents them from drying out so that rep-
tiles can lay their eggs on land. Like most
fishes, reptiles are poikilotherms and 
ectotherms, commonly called “cold-
blooded.” Like other poikilotherms, their
metabolic rate—and therefore activity
level—varies with temperature: They get
sluggish in the cold. This tends to keep
them out of cold regions, especially on
land because the air temperature fluctu-
ates more widely than does the ocean
temperature.
•
Reptiles are air-breathing, ectothermic (“cold-
blooded”), poikilothermic vertebrates. Their
skin is covered with dry scales and nearly all
lay their eggs on land.
•
Reptiles first appeared more than
300 million years ago, and several differ-
ent groups have invaded the seas. Many
are long gone, like the ichthyosaurs (see
Fig. 9.14) that thrived during the so-
called Age of Reptiles. Only a few rep-
tiles still roam the seas. Some are rare and
endangered; others, however, are com-
mon and widely distributed.
Sea Turtles
Sea turtles belong to an ancient group of
reptiles. Their bodies are enclosed by an
armor-like shell, or carapace, that is fused
to the backbone. Unlike land tortoises
and turtles, sea turtles cannot retract their
heads into the shell. Their legs, particu-
larly the larger forelimbs, are modified
into flippers for swimming.
There are only nine species of sea
turtles, which live primarily in warm wa-
ters. Green turtles (Chelonia mydas, see
photo on page 182) were once found in
coastal waters throughout the tropics.
Their shells may grow to 1 m (40 in) in
length. They feed mostly on seagrasses
and seaweeds. Like all turtles, green tur-
tles lack teeth, but they have strong biting
jaws. The hawksbill turtle (Eretmochelys
imbricata; Fig. 9.2a) is smaller, and the
shell is reddish brown with yellow
streaks. It uses its beak-like mouth to
feed on encrusting animals (sponges, sea
squirts, barnacles) and seaweeds.
The largest sea turtle is the leatherback
(Dermochelys coriacea; Fig. 9.2b). Individuals
may attain a length of 2 m (7 ft) and weigh
at least 540 kg (1,200 lb). Instead of a solid
shell, they have a series of small bones
buried in the dark skin, forming distinct
longitudinal ridges. Leatherbacks are an
open-water, deep-diving species and are
rarely seen except on nesting beaches.
Their diet consists largely of jellyfishes.
All sea turtles must return to land to
reproduce. They migrate long distances
to lay their eggs on remote sandy
beaches, and were doing so millions of
years before humans appeared on the
scene. Green turtles still gather to nest on
beaches on the east coast of Central
America, Northern Australia, Southeast
Asia, Ascension Island (in the middle of
the South Atlantic), and a few other loca-
tions. Marine biologists have tagged adult
sea turtles at Ascension and have found
that the turtles regularly cross 2,200 km
(1,360 mi) of open water to their feeding
grounds along the coast of Brazil, a jour-
ney that takes a little more than two
months (see world map in Appendix B).
Though we are still not sure how they
find their way, evidence suggests that
they do it by sensing wave motion and
the earth’s magnetic field.
Most of what we know about the re-
production of sea turtles is based on the
green turtle. They return to their nesting
areas every two to four years, often
against prevailing currents. Evidence that
femalesreturn to the beaches where they
were born has been obtained by analyzing
the DNA of breeding populations at sep-
arate Caribbean and Atlantic sites. The
DNA of turtles breeding in one area dif-
fers from the DNA of turtles breeding at
Chapter 9 Marine Reptiles, Birds, and Mammals 181
Poikilotherms Organisms that have a
body temperature that varies with that 
of the environment.
Ectotherms Organisms that lose
metabolic heat to the environment
without it affecting the body temperature.
Chapter 4, p. 80
DNA A complex molecule that
contains a cell’s genetic information.
Chapter 4, p. 70
(a)
(b)
FIGURE 9.2 (a) The hawksbill turtle
(Eretmochelys imbricata) takes its name from
the shape of its jaw (see Fig. 4.15). It is the
source of tortoiseshell. (b) The largest of all
sea turtles, the leatherback turtle (Dermochelys
coriacea), sometimes ventures into cold waters
as far north as Newfoundland and Alaska.
Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
other sites. It thus appears that turtles
keep returning to the same place genera-
tion after generation.
Copulating pairs of sea turtles are
often seen offshore, but only females ven-
ture ashore, usually at night. Therefore
biologists have mostly tagged females,
because turtles can be tagged most easily
on land. The females congregate on the
beach, and each proceeds to excavate a
hole in the sand using both pairs of flip-
pers (Fig. 9.3). They lay between 100 and
160 large, leathery eggs in this nest. The
female covers the eggs with sand before
she returns to the sea. She may make sev-
eral trips ashore during the breeding sea-
son, laying eggs each time.
The eggs hatch after about 60 days
of incubation in the sand. The baby tur-
tles must then dig themselves out of the
sand and crawl all the way back to the
water, protected by darkness if they’re
lucky. Green turtles and other sea turtles
have many enemies. The eggs are often
eaten by dogs, ghost crabs, wild pigs, and
other animals. The hatchlings are easy
prey for land crabs and birds, especially
during the day. Even more young turtles
are lost in the water, where they are taken
by a variety of fishes and seabirds.
Sea Snakes
Approximately 55 species of sea snakes
are found in the tropical Indian and Pa-
cific oceans (Fig. 9.4). Their bodies are
laterally flattened, and the tail paddle-
shaped for swimming. Most are 1 to 1.3 m
(3 to 4 ft) long. Practically all sea snakes
lead a totally marine existence. They mate
in the ocean and are ovoviviparous, giving
birth to live young. A few species, how-
ever, still come ashore to lay their eggs.
Like all snakes, sea snakes are carni-
vores. Most feed on bottom fish, a few
specializing in fish eggs. They are closely
182 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
Humans are by far the most formidable and destructive enemies of
sea turtles. Many nesting areas have been turned into resorts or pub-
lic beaches. Females searching for nesting beaches avoid lights be-
cause dark areas along the horizon indicate land, and lights along a
beach look like a starry horizon. Artificial lighting also disorients
baby turtles after hatching so they do not head to sea and therefore
die. Turtles drown in fishing nets, especially drift nets, and choke to
death after swallowing plastic bags they think are jellyfishes. Turtles
have been used as food for centuries. Their eggs are taken by the
bucketful, and are located by pushing a stick into the sand until it
comes out yellow. The eggs are eaten or fed to pigs or cattle. Turtle
eggs, particularly those of leatherbacks, are said to be an aphrodisiac,
a myth that probably arose because adult turtles can be seen copulat-
ing for long periods at sea.
Sea turtles can live for months without food or water. In the
days before refrigeration, sailors kept them alive aboard ship as a
source of fresh meat, storing them on their backs for months. Com-
ing ashore by the thousands, females were an easy catch. They were,
and still are, immobilized by turning them on their backs to be
gathered later without giving them a chance to lay their eggs. The
green turtle is especially esteemed for its meat, and its cartilage is
used to make turtle soup. Some people consider the oily leatherback
meat a delicacy.
The polished shell of the hawksbill is the source of valuable
tortoiseshell used to make jewelry, combs, and other articles (see
Fig. 18.15), particularly in Japan. Sea turtle leather, which is soft
and durable, is much prized for shoes, handbags, and wallets.
Leather articles from illegally slaughtered animals still make their
way into the United States from Mexico and other countries. The
oil of many sea turtles is also of commercial value. Even baby sea
turtles are valuable; they are stuffed and sold as souvenirs.
The green turtle, once very common, has disappeared in many
areas as the result of relentless overexploitation for eggs and meat. It
is the most widely distributed and most common of all sea turtles,
but only an estimated half a million individuals are left worldwide.
All sea turtle species are classified as threatened worldwide be-
cause their numbers are low (see Table 18.1, p. 422). For example,
only about 4,000 nesting leatherback turtles remain in the Pacific
Ocean. Sea turtles are not protected at all in many countries, and in
those where they are, enforcement is difficult. Shrimp nets are esti-
mated to kill up to 4,000 sea turtles a year in Southeast Asia. Many
more become entangled in gill and drift nets and die asphyxiated. It is
impossible to protect all coasts and nesting grounds from fishers and
egg hunters. Stricter worldwide enforcement of conservation practices,
the control of pollution, the regulation of trade in sea turtle products,
and the restocking of former nesting areas might help save them.
All six species of sea turtles in the United States are protected
under the Endangered Species Act of 1973. Three of these species are
classified as threatened. Three other species are classified as endan-
gered and as such in great danger of disappearing: the leatherback,
hawksbill, and Kemp’s (or Atlantic) ridley (Lepidochelys kempii).
Shrimp nets in the Gulf of Mexico have been especially deadly to the
Kemp’s ridley turtle, once very common but now so rare that only a
few hundred breeding females remain. It is the most endangered of all
sea turtles. After a lengthy struggle, the U.S. government mandated
that shrimp nets be fitted with turtle exclusion devices, or TEDs, that
allow sea turtles to escape once caught in the nets.
THE ENDANGERED SEA TURTLES
Green sea turtle (Chelonia mydas).
Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
related to cobras and their allies, the most
venomous of all snakes. Sea snakes are
among the most common of all ven-
omous snakes, and their bites can be fatal
to humans. Fortunately, they are rarely
aggressive, and the mouth is too small to
get a good bite. Most casualties, swim-
mers accidentally stepping on them and
fishers removing them from nets, have
been reported in Southeast Asia. Sea
snakes are also victims of overexploita-
tion. They are hunted for their skins, and
some species have become rare.
Other Marine Reptiles
An unusual lizard is among the unique in-
habitants of the Galápagos Islands, which
lie off the Pacific coast of South America.
The marine iguana (Amblyrhynchus crista-
tus; Fig. 9.5) spends most of its time bask-
ing in large groups on rocks along the
coast, warming up after swimming in the
cold water. It eats seaweeds and can dive
as deep as 10 m (33 ft) to graze.
Chapter 9 Marine Reptiles,Birds, and Mammals 183
FIGURE 9.3 Egg laying in green turtles
(Chelonia mydas) culminates a long and
hazardous trip by females. It is at this time 
that they are most vulnerable to egg collectors.
This photograph was taken on Sipadan Island,
one of the Turtle Islands off the northeastern
coast of Borneo, Malaysia.
Ovoviviparous Animals Animals in
which the eggs develop and hatch in the
reproductive tract of females.
Chapter 8, p. 176
Homeotherms Organisms able to keep
body temperature more or less constant
regardless of the temperature of the
environment.
Endotherms Organisms that retain
some metabolic heat, which raises their
body temperature.
Chapter 4, p. 81
FIGURE 9.4 Sea snakes are found from the Indian Ocean coast of South Africa to the
Pacific coast of tropical America, where they occur from the Gulf of California to Ecuador. They
sometimes occur under floating debris, feeding on the fish it attracts. The conspicuous coloration of
sea snakes may be a warning to potential predators because many fishes learn to associate the bright
colors with danger. There are no sea snakes in the Atlantic, but a sea-level canal across Central
America may allow their migration into the Caribbean.
The other marine reptile is the salt-
water crocodile (Crocodylus porosus; see 
Fig. 17.17), which inhabits mangrove
swamps and estuaries in the Eastern Indian
Ocean, Australia, and some of the Western
Pacific islands. They live mostly on the
coast but are known to venture into the
open sea. There is a record of an individual
10 m (33 ft) long, but they are rarely over 
6 m (20 ft). They are among the most ag-
gressive of all marine animals and are
known to attack and eat people. Where they
occur, they are more feared than sharks.
•
Marine reptiles include the sea turtles, sea
snakes, the marine iguana, and the saltwater
crocodile.
•
SEABIRDS
Birds have some significant advantages
over reptiles, including the ability to fly.
Birds are homeotherms, commonly re-
ferred to as “warm-blooded.” They are
also endotherms. This has allowed them
to live in a wide variety of environments.
Their bodies are covered with waterproof
feathers that help conserve body heat.
Waterproofing is provided by oil from a
gland above the base of the tail. The
birds preen by rubbing the oil into their
feathers with their beaks. Flight is made
Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
easier by their light, hollow bones. Fur-
thermore, their eggs have hard shells that
are more resistant to water loss than
those of reptiles.
•
Birds are endothermic (“warm-blooded”),
homeothermic vertebrates that have feathers
and light bones as adaptations for flight.
•
Seabirds are those birds that spend a
significant part of their lives at sea and
feed on marine organisms. Seabirds nest
on land. Most breed in large colonies,
mate as lifelong pairs, and take care of
their young. True seabirds have webbed
feet for swimming.
Seabirds descended from several dif-
ferent groups of land birds. As a result,
they differ widely in their flying skills,
feeding mechanisms, and ability to live
away from land.
•
Seabirds are birds that nest on land but feed
entirely or partially at sea.
•
Though comprising only about 3%
of the estimated 9,700 species of birds,
seabirds are distributed from pole to pole,
and their impact on marine life is signifi-
cant. Most are predators of fish, squid,
and bottom invertebrates, but some feed
on plankton. Seabirds have amazing ap-
petites. They need a lot of food to supply
the energy required to maintain their
body temperatures.
Penguins
Penguins (Fig. 9.6a) are the seabirds
most fully adapted for life at sea. They
are flightless, with wings modified into
stubby “flippers” that allow them to “fly”
underwater. Their bones are denser than
those of other birds to reduce buoyancy
and make diving easier.
184 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
(a) (b)
(c)
FIGURE 9.6 (a) An emperor penguin
(Aptenodytes forsteri) and chick. The emperor
is the largest living penguin, with a height of
up to 115 cm (45 in). (b) The brown booby
(Sula leucogaster) nests in the Caribbean and
Gulf of California. It is a regular visitor to the
Gulf of Mexico. (c) The gannet (Morus
bassanus) is the largest seabird in the North
Atlantic. It nests in large colonies on offshore
islands such as Bonaventure Island in Quebec,
Canada, where around 50,000 birds breed
every year.
FIGURE 9.5 The marine iguana of the
Galápagos Islands (Amblyrhynchus cristatus) is
probably one of the ugliest creatures of the
sea, with the face of a dragon and peeling
skin. In the water, however, these iguanas are
elegant swimmers. They swim by undulating
the body and the laterally flattened tail, the tip
of which is shown on the bottom left.
Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
Penguins are spectacular swim-
mers, propelling their streamlined bod-
ies with powerful strokes of the wings
(see Fig. 9.14). They can also jump out
of the water and sometimes cover long
distances by alternately swimming and
jumping. On land it is another story:
They are clumsy and awkward. They are
nearsighted, having eyes that are
adapted for underwater vision.
Penguins are also adapted for cold
temperatures. Protection against low
temperatures is provided by a layer of fat
under the skin. The dense, waterproof
feathers trap air that, warmed by body
heat, protects against the cold like a
down coat. All but one of the 18 species
of penguins live primarily in Antarctica
and other cold regions of the Southern
Hemisphere. The exception is the Galá-
pagos penguin (Spheniscus mendiculus),
which lives right on the Equator. Even
so, this penguin is confined to regions
that are bathed by cold currents.
The larger penguins, like the impos-
ing emperor penguin (Aptenodytes forsteri;
Fig. 9.6a), hunt for fish and squid. The
Adélie (Pygoscelis adeliae) and other small
penguins feed mostly on krill. Penguins
have strong beaks, a characteristic of
seabirds that feed on fish and large
plankton like krill (Fig. 9.7b). Some
species migrate seasonally between feed-
ing grounds at sea and nesting areas on
land or ice. They establish breeding
colonies, which in Adélies may number
more than a million pairs.
Breeding season and number of eggs
laid vary from species to species. Emperor
penguins mate for life. The male incu-
bates a single large egg during the dark
Antarctic winter. The female leaves to
feed as soon as she lays the egg. The
male, standing on ice, must keep the egg
warm by holding it on top of his feet and
against his body for 64 days. Males hud-
dle together to protect themselves from
the cold and the dreadful winter storms.
You may wonder why the penguins
lay their eggs at the coldest time of the
year. Reproduction is timed so that the
egg hatches during the productive
Antarctic summer, when food is most
plentiful. When the egg hatches, the fe-
male finally returns and regurgitates food
for the fuzzy chick. After that, both par-
ents take turns feeding the chick. While
the parents feed, the fast-growing young
are herded into groups guarded by a few
adult “babysitters.” Returning parents
identify their chick among thousands by
its voice and appearance. The parents
continue to feed the chick for five and a
half months, until it is strong enough to
feed itself at sea.
Tubenoses
The tubenoses comprise a large group of
seabirds with distinctive tube-like nostrils
and heavy beaks that are usually curved at
the tip (Fig. 9.7a). They spend months
and even years on the open sea.Like other
seabirds and sea turtles, they have salt
glands that get rid of excess salts; these
empty into the nostrils. Tubenoses include
the albatrosses (Diomedea), shearwaters
(Puffinus), and petrels (Pterodroma).
Tubenoses are very skillful fliers. Most
catch fish at the sea surface (Fig. 9.7a),
though some scavenge on dead birds 
or whales. The whalebirds, or prions
(Pachyptila), feed on krill and other plank-
ton. Albatrosses are magnificent gliders
with huge wings that hardly ever seem to
flap. Wandering albatrosses (D. exulans)
and royal albatrosses (D. epomophora) have
wingspans of up to 3.4 m (11 ft), the
longest of any bird alive.
Chapter 9 Marine Reptiles, Birds, and Mammals 185
Krill Planktonic, 
shrimp-like crustaceans.
Chapter 7, p. 137; Figure 17.12
(a) (b) (c)
(d)
FIGURE 9.7 The shape of a seabird’s beak is related to the kind of food it eats and the bird’s
feeding style. (a) In tubenoses such as petrels (Pterodroma), the beak is relatively short, heavy, and
hooked—an ideal shape for holding and tearing prey that is too big to be swallowed whole. Such a
beak is best suited for shallow feeding because its size and shape interfere with fast pursuit
underwater. (b) The beak is heavy but more streamlined in the penguin (Aptenodytes and others), the
razorbill (Alca), and other seabirds that dive deeper to feed on crustaceans and other prey. (c) Boobies
(Sula), terns (Sterna), and other plunge divers have a straight and narrow beak for feeding on fish that
are swallowed whole. (d) Skimmers (Rynchops) are the only birds with a lower part of the beak that is
longer than the upper, which permits feeding while flying. Shorebirds that feed on mudflats have a
long, thin beak that allows them to get to prey buried in the mud (see Fig. 12.12).
Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
Male and female tubenoses remain
faithful to each other and perform elabo-
rate courtship and greeting behaviors.
Most nest on remote islands, on cliffs
that are inaccessible to predators. Incuba-
tion and care of the single chick takes
eight months, and even longer in some
species. Tubenoses make some of the
most spectacular migrations of any ani-
mal. Many breed on islands around
Antarctica, then migrate across the open
ocean to summer feeding grounds near
the Arctic. The wandering albatross gets
its name from the fact that it spends two
years or more traveling around the
Southern Hemisphere before returning to
nesting sites near Antarctica. Some non-
breeding individuals wander off and pay
visits as far away as California and the
Mediterranean!
Pelicans and Allies
Several quite different-looking seabirds are
grouped together because they have web-
bing between all four toes. They are rela-
tively large fish-eaters of wide distribution.
Pelicans (Pelecanus) have a unique
pouch below their large beaks. Some
species, like the brown pelican (P. occiden-
talis; see Fig. 18.9), catch their food by
plunging into the water and catching fish
in the pouch (Fig. 9.8). The brown pelican
was once common along the coasts of the
United States but was decimated by pesti-
cide pollution (see “Toxic Chemicals,”
p. 415). It has made a comeback as a result
of restrictions on the manufacture and use
of the pesticide DDT. Cormorants (Pha-
lacrocorax) are black, long-necked seabirds
that dive and pursue their prey. They can
be easily identified by their low flights over
water and the fact that they float low in the
water, with only the neck above the sur-
face. Frigate birds (Fregata) have narrow
wings and a long, forked tail. They soar
majestically along the coast, forcing other
seabirds to regurgitate fish in midair or
catching prey from the surface (Fig. 9.8).
These agile pirates seldom enter the water,
not even to rest, because their feathers are
not very waterproof.
Pelicans and related species nest in
large colonies along the coast. They build
messy nests of twigs and anything else
they can find. The excrement of millions
of boobies (Fig. 9.6b), cormorants, peli-
cans, and other seabirds accumulates as
guano. Guano deposits are particularly
thick in dry coastal regions and islands
near very productive waters, such as the
coasts of Perú, Chile, and southwest
Africa. These deposits are mined for fer-
tilizer (see “Of Fish and Seabirds, Fishers
and Chickens,” p. 393).
Gulls and Allies
Gulls (Larus) and their kin make up the
largest variety of seabirds. Common and
widespread, gulls are predators and scav-
engers happy to eat just about anything
(Fig. 9.8). They are very successful in the
company of humans and congregate near
piers, garbage dumps, or anywhere else
we throw refuse. Jaegers (Stercorarius) and
skuas (Catharacta) are gull-like predators
that steal fish from other birds (Fig. 9.8).
They nest near the rookeries of penguins
and other seabirds and eat their eggs and
young.
186 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
Surface plunging
(pelicans)
Aerial pursuit
(jaegers)
Surface plunging
(boobies)
Dipping (gulls)
Aerial pursuit
(frigate birds)
Pursuit plunging
(shearwaters)
Pursuit diving
with feet
(cormorants)
(penguins)
(diving petrels)
Pursuit diving
with wings
Pattering
(storm petrels)
FIGURE 9.8 Feeding strategies vary widely among seabirds. Pelicans (Pelecanus) and boobies (Sula) plunge into the water, jaegers (Stercorarius)
pursue other seabirds and force them to regurgitate food, and frigate birds (Fregata) take fish from the surface and steal fish from other seabirds. Gulls
(Larus) rarely dive from the air, and storm petrels (Oceanodroma) simply flutter over the waves. Divers such as cormorants (Phalacrocorax) pursue prey
underwater, swimming with their wings or feet. Mudflat shorebirds also follow various strategies (see Fig. 12.13).
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Terns (Sterna) are graceful flyers
that hover over their prey before plunging
for it. Their slender beaks are specialized
to catch small fish, which they swallow
whole (Fig. 9.7c). The Arctic tern (S. par-
adisaea) is another amazing wanderer. It
breeds in the Arctic during the northern
summer, travels 16,000 km (10,000 mi)
to Antarctica for the southern summer,
and then returns to the Arctic.
Also related to gulls are several cold-
water diving seabirds. Puffins (Fratercula)
have heavy beaks that make them look
like misplaced parrots. The related razor-
bill (Alca torda) is a black and white bird
reminiscent of penguins (Fig. 9.7b). In
fact, these birds may fill the role of pen-
guins, which are absent in the Northern
Hemisphere. Like penguins, they use
their wings to swim underwater. Their ex-
tinct cousin, the great auk (Pinguinus im-
pennis), looked and acted like a penguin.
Great auks once lived in great numbers in
the North Atlantic but were slaughtered
for their eggs, meat, and feathers. The last
great auk died in 1844.
Shorebirds
Usually included among the seabirds are
many species of wading shorebirds that
do not have webbed feet. Because they do
not swim much, they are not really
seabirds in the strict sense. Many live in
inland waters, as well as the sea. Some are
common in estuaries and coastal marshes.
Plovers, sandpipers, and similar birds are
related to gulls (see Fig. 9.1). Many other
shorebirds may live on the coast: rails,
coots, herons, egrets, and even ducks.
The distribution and significance of
shorebirds in estuaries will be discussed
in Chapter 12.
MARINE MAMMALS
About 200 million years ago another
major group of air-breathing vertebrates,
the mammals (class Mammalia), evolved
from now-extinctreptiles. For a long
time the mammals were overshadowed
by the dinosaurs, which were reptiles.
About 65 million years ago, however, the
dinosaurs disappeared. It was then that
mammals thrived, taking the place of the
dinosaurs. There are now roughly 4,600
species of mammals, including humans.
Fishes, reptiles, and birds each outnum-
ber mammals in number of species.
Like birds, mammals have the ad-
vantage of being endotherms, or “warm-
blooded,” and homeotherms. The skin of
mammals, however, has hair instead of
feathers to retain body heat. With few
exceptions, mammals are viviparous.
The embryo receives nutrients and oxy-
gen through the placenta, a membrane
that connects it to the womb. It is also
known as the afterbirth. The newborn is
fed by milk secreted by the mother’s
mammary glands. Instead of releasing
millions of eggs, mammals produce
few—but well-cared-for—young.
And then there is the brain. It is
larger in relation to body size and far
more complex than that of other verte-
brates, allowing the storage and process-
ing of more information. This accounts
in part for the amazing adaptability of
mammals. They live anywhere there is 
air to breathe and food to eat. This, of
course, includes the ocean.
Types of Marine Mammals
There is something fascinating about
mammals that live at sea like fishes. At
least five different groups of land mammals
succeeded in invading the oceans. They
have followed different paths in adapting
to the marine environment. Some are so
fish-like that we have to remind ourselves
that they have hair and bear live young
nourished by their mother’s milk.
Seals, Sea Lions, and Walruses
Seals and related forms are marine mam-
mals that have paddle-shaped flippers for
swimming but still need to rest and breed
on land. They make up one of the 19 or
20 major groups, or orders, of mammals,
the pinnipeds (order Pinnipedia; see
Fig. 9.1). Pinnipeds evolved from an early
form of terrestrial carnivore (order Car-
nivora), which includes cats, dogs, bears,
and their kin. The similarities are so close
that many scientists classify them with
the carnivores. Pinnipeds are predators,
feeding mostly on fish and squid. Their
streamlined bodies are adapted for swim-
ming (Fig. 9.9).
Most pinnipeds live in cold water.
To keep warm they have a thick layer of
fat under their skin called blubber. Be-
sides acting as insulation, it serves as a
food reserve and helps provide buoyancy.
Pinnipeds also have bristly hair for added
protection against the cold. Many of
them are quite large, which also helps
conserve body heat because large animals
have less surface area for their size than
small animals, and therefore lose less
body heat (see Fig. 4.17).
•
Pinnipeds, which include seals and their rela-
tives, are marine mammals with flippers and
blubber that need to breed on land.
•
The largest group of pinnipeds, in-
cluding some 19 species, is the seals.
Seals are distinguished by having rear
flippers that cannot be moved forward
(Fig. 9.9b). On land they must move by
pulling themselves along with their front
flippers. They swim with powerful
strokes of the rear flippers.
Harbor seals (Phoca vitulina) (Fig. 9.9b)
are common in both the North Atlantic
and North Pacific. Elephant seals (Mir-
ounga; Fig. 9.10a) are the largest pin-
nipeds. Males, or bulls, reach 6 m (20 ft) in
length and can weigh as much as 3,600 kg
(4 tons). One unusual seal is the crabeater
seal (Lobodon carcinophagus), which actually
feeds on Antarctic krill. These seals strain
krill from the water with their intricately
cusped, sieve-like teeth. Unlike most seals,
monk seals (Monachus) live in warm re-
gions. The Mediterranean (M. monachus)
and Hawaiian (M. schauinslandi) monk
seals are now endangered. A third species,
the Caribbean monk seal (M. tropicalis),
was last seen in 1952.
Seals have been hunted for their skin
and meat, and for the oil extracted from
their blubber. The Marine Mammal Pro-
tection Act of 1972 extends protection to
all marine mammals and restricts the sale
Chapter 9 Marine Reptiles, Birds, and Mammals 187
Viviparous Animals Live-bearing
animals whose embryos develop within
their mothers’ bodies and are nourished
by the maternal bloodstream.
Chapter 8, p. 177
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External ear
Long neck,
able to turn
in water
External
testicles
in males
Posterior flippers
can be moved
forward Anterior flippers rotatebackward to support
weight and keep head
erect; edge not covered
with hair or nails to
reduce water resistance
in swimming
Uses anterior
flippers in swimming
Sea Lion
No external ear
Uses posterior flippers
in swimming; they cannot
be moved forward
Anterior flippers covered
with hair, five toes with
sharp nails; they cannot be
rotated backward
No external
testicles in male
Short neck
Seal
(a) (b)
FIGURE 9.9 Though they differ in some structural features and the ways in which they swim and move on land, sea lions (a) and seals (b) are now
thought to have evolved from the same group of land carnivores.
FIGURE 9.10 Seals. (a) The northern elephant seal (Mirounga angustirostris), so-called
because of the huge proboscis of the male, was almost exterminated for its blubber. By 1890 only
about 100 remained, but because of protection and a drop in the use of its blubber it rapidly
recovered, and there are now more than 100,000 of them in California and Baja California. (b) The
New Zealand fur seal (Arctocephalus forsteri), like the other fur seals, is characterized by its thick
underfur. (c) Female harp seals (Phoca groenlandica), one seen here peeking through the ice, give birth
to white, furry pups on the floating Arctic ice. Pups must grow fast and shed their white coats before
the drifting ice melts. In eastern Canada the clubbing of young pups to harvest their white fur
provoked worldwide protests, and Canada banned the sale of the fur. Unemployment caused by the
collapse of fisheries, however, prompted the government to reverse itself and the hunt has resumed.
(a) (c)
(b)
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of their products in the United States.
For some seals, this protection has not
been enough (see Table 18.1, p. 422).
Sea lions, or eared seals, are similar
to seals, except that they have external ears
(Fig. 9.9a). They can also move their rear
flippers forward, so they can use all four
limbs to walk or run on land. The front
flippers can be rotated backward to sup-
port the body, permitting the animal to sit
on land with its neck and head raised. Sea
lions are graceful and agile swimmers, re-
lying mostly on their broad front flippers.
Adult males are much bigger than females,
or cows, and have a massive head with a
hairy mane (see Fig. 9.32a). The head of
sea lions looks dog-like, whereas the head
of seals has much softer outlines, making
it look more like a cat’s (Fig. 9.9).
There are five species of sea lions, plus
nine species of the related fur seals. The
most familiar of all is the California sea lion
(Zalophus californianus; see Fig. 9.33) of 
the Pacific coast of North America and the
Galápagos Islands. These sea lions are the
trained barking circus “seals” that do tricks
for a fish or two. Fur seals (Fig. 9.10b), like
the northern fur seal (Callorhinus ursinus),
were once almost exterminated for their
thick fur. They are now mostly protected
around the world, though some species are
still hunted. Sea lions were luckier because
they lack the underfur of theircousins. Still,
both sea lions and fur seals may run afoul of
fishers. They sometimes drown in nets or
are shot because of their notorious ability to
steal fish.
The walrus (Odobenus rosmarus;
Fig. 9.11) is a large pinniped with a pair
of distinctive tusks protruding down from
the mouth. It feeds mostly on bottom in-
vertebrates, particularly clams. It was once
thought that the walrus used its tusks to
dig up food, but there is no evidence for
this. Instead, these pinnipeds apparently
suck up their food as they move along the
bottom. The stiff whiskers of the snout
probably act as feelers. The tusks are used
for defense, and to hold or anchor to ice.
Sea Otters and Polar Bears
Though there is doubt about the pinnipeds,
the sea otter (Enhydra lutris; Fig. 9.12) 
is definitely a member of the order Car-
nivora. The sea otter is the smallest marine
mammal; an average male weighs 25 to 
35 kg (60 to 80 lb). It also differs from
other marine mammals in lacking a layer of
blubber. Insulation from the cold is pro-
vided by air trapped in its dense fur. This
splendid, dark brown fur unfortunately at-
tracted hunters. Sea otters were slaughtered
to near extinction until they became pro-
tected by an international agreement in
1911. The sea otter was then able to slowly
expand from the few individuals that had
managed to survive in some remote loca-
tions. Their numbers, however, have lev-
eled off and the species is still endangered
(see Table 18.1, p. 422).
Sea otters are playful and intelligent
animals. They spend most or all of their
time in the water, including breeding and
giving birth. The furry pup is constantly
groomed and nursed by its mother. Sea
otters require 7 to 9 kg (15 to 20 lb) of
Chapter 9 Marine Reptiles, Birds, and Mammals 189
FIGURE 9.11 Walruses (Odobenus rosmarus) typically inhabit the edge of pack ice in the
Arctic. They migrate as far south as the Aleutian Islands and Hudson Bay, Canada. They also
crowd onto beaches on isolated islands that they use as resting places. The walrus is still hunted
legally by native Alaskans and Siberians.
FIGURE 9.12 Sea otters (Enhydra lutris) are remarkable for their use of a tool—a rock for
crushing shells. They carry a flat rock in side “pockets” of loose skin and fur. The sea otter floats on
its back at the surface, places the rock on its chest, and crushes its toughest prey against it. Some
have been observed carrying and using beer bottles as tools!
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food every day, so they spend a lot of
time looking for it. They satisfy their rav-
enous appetites with sea urchins, abalone,
mussels, crabs, other invertebrates, and
even fishes. They live in or around kelp
beds from the Pacific coast of Siberia to
central California. Sea otters help protect
kelp beds from sea urchins (see “Kelp
Communities,” p. 290).
The polar bear (Ursus maritimus) is
the second member of the order Car-
nivora that inhabits the marine environ-
ment. Polar bears are semiaquatic animals
that spend a good part of their lives on
drifting ice in the Arctic. They feed pri-
marily on seals, which they stalk and cap-
ture as the seals surface to breathe or rest.
Manatees and Dugongs
It is hard to believe that relatives of the
elephant live at sea. Manatees and the
dugong are also known as sea cows, or
sirenians (order Sirenia). They have a
pair of front flippers but no rear limbs
(Fig. 9.13). They swim with up-and-
down strokes of the paddle-shaped, hori-
zontal tail. The round, tapered body is
well padded with blubber. They have
wrinkled skin with a few scattered hairs.
The group is named after the sea nymphs
or mermaids (sirenas in Spanish) whose
songs drove sailors crazy!
Sirenians are gentle, peaceful crea-
tures. They usually live in groups. They are
the only strict vegetarians among marine
mammals. Their large lips are used to feed
on seagrasses and other aquatic vegetation.
All sirenians are large. Dugongs may reach
3 m (10 ft) in length and 420 kg (930 lb)
in weight. Manatees reach 4.5 m (almost
15 ft) and 600 kg (1,320 lb) in weight. The
largest sirenian of all was the now-extinct
Steller’s sea cow, which supposedly grew to
7.5 m (25 ft) long (see Fig. 18.13).
Humans have exploited sirenians for
their meat (which supposedly tastes like
veal), skin, and oil-rich blubber. Like ele-
phants and other large mammals, they
reproduce slowly, typically one calf every
three years. Only four species remain,
and all are in danger of extinction (see
Table 18.1, p. 420). Three species of
manatees (Trichechus) live in the Atlantic
Ocean; one is restricted to the Amazon,
and the others inhabit shallow coastal
waters and rivers from Florida to West
Africa. The dugong (Dugong dugon) is
strictly marine and survives from East
Africa to some of the western Pacific is-
lands. Its numbers are critically low
throughout most of its range.
Whales, Dolphins,
and Porpoises
The largest group of marine mammals is
the cetaceans (order Cetacea), the
whales, dolphins, and porpoises. No
group of marine animals has captured our
imaginations like the dolphins and
whales. They have inspired countless leg-
ends and works of art and literature (see
“Oceans and Cultures,” p. 430). The res-
cue of whales stranded on a beach or the
birth of a killer whale in an oceanarium
brings out strong emotions in all of us.
Of all marine mammals, the ceta-
ceans, together with the sirenians, have
made the most complete transition to
aquatic life. Whereas most other marine
mammals return to land at least part of the
time, these two groups spend their entire
lives in the water. The bodies of cetaceans
are streamlined and look remarkably fish-
like (Fig. 9.14). This is a dramatic example
of convergent evolution, where different
species develop similar structures because
they have similar lifestyles. Though they
superficially resemble fishes, cetaceans
breathe air and will drown if trapped
below the surface. They are “warm-
blooded,” have hair (though scanty), and
produce milk for their young.
Cetaceans have a pair of front flip-
pers (Fig. 9.15), but the rear pair of
limbs has disappeared. Actually, the rear
limbs are present in the embryo but fail
to develop (Fig. 9.16). In adults they re-
main only as small, useless bones. Like
fishes, many cetaceans have a dorsal fin.
The muscular tail ends in a pair of fin-
like, horizontal flukes. Blubber (see
Fig. 4.2) provides insulation and buoy-
ancy; body hair is practically absent.
Cetacean nostrils differ from those of
other mammals. Rather than being on
the front of the head, they are on top,
forming a single or double opening
called the blowhole (Fig. 9.15).
There are around 90 species of
cetaceans. They are all marine except for
five species of freshwater dolphins.
Cetaceans are divided into two groups:
(1) the toothless, filter-feeding whales
and (2) the toothed, carnivorous whales, 
a group that includes the dolphins and
porpoises.
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FIGURE 9.13 It has been estimated that approximately 1,000 West Indian manatees
(Trichechus manatus) remain along the coasts and rivers of Florida. Some concentrate in the warm-
water effluents of power plants. They are strictly protected, but collisions with boats take their toll.
Manatees have been considered as a possible way to control weeds that sometimes block waterways.
Some people have suggested raising them for food.
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Chapter 9 Marine Reptiles, Birds, and Mammals 191
Shark
Penguin
Dolphin
IchthyosaurFIGURE 9.14 Streamlining to reduce water resistance evolved independently in different groups of fast-swimming marine animals: sharks,
ichthyosaurs (reptiles that became extinct about 65 million years ago), dolphins, and penguins. Notice that dolphins lack posterior fins and that their
flukes are horizontal, not vertical like the tail (caudal fin) of fishes (see Fig. 9.24).
Blowhole Eye Ear opening
Dorsal fin
Fluke
Side View 
Ventral View
Dorsal View
Blowhole Dorsal fin
Navel Genital slit
Anus
Flipper
Baleen
Slit for nipple
of mammary gland
FIGURE 9.15 External morphology of the blue whale (Balaenoptera musculus). A female is
shown; males have a genital slit halfway between the anus and navel, and they lack mammary slits.
Rear limb
Umbilical cord
Tail
FIGURE 9.16 The fetus of a white-
sided dolphin (Lagenorhynchus) shows two
distinct pairs of limbs; the rear pair will
eventually disappear. The umbilical cord
connects the fetus with the placenta.
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The toothless whales are better
known as the baleen whales. Instead of
teeth they have rows of flexible, fibrous
plates named baleen that hang from the
upper jaws (Fig. 9.17). Baleen is made of
keratin, the same material as our hair and
nails. The inner edge of each plate con-
sists of hair-like bristles that overlap and
form a dense mat in the roof of the
mouth. The whale filter feeds by taking a
big mouthful of water and squeezing it
out through the bristles. The whale then
licks off the food that is left behind on
the bristles and swallows it.
•
Baleen whales are cetaceans that filter feed
with baleen plates.
•
Baleen whales are not only the
largest whales, they are among the
largest animals that have ever lived on
earth. There are 11 species of these ma-
jestic creatures. They were once common
in all the oceans, but overhunting has
brought many species to the brink of ex-
tinction. The blue whale (Balaenoptera
musculus), which is actually blue-gray, is
the largest of all (Fig. 9.18). Males aver-
age 25 m (80 ft), and there is a record of
a female 33.5 m (110 ft) long. How do
you weigh a blue whale? Very carefully—
they average 80,000 to 130,000 kg (90 to
140 tons), but the record is an estimated
178,000 kg (200 tons)!
The blue whale, the fin whale (B. phy-
salus; Fig. 9.18), and the minke whale 
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It should be obvious that cetaceans are mammals. Their streamlined
bodies, absence of hind legs, and the presence of a fluke and blow-
hole cannot disguise their affinities with land-dwelling mammals.
Unlike the cases of sea otters and pinnipeds, however, it is not easy
to suggest what the first whales looked like. Extinct but already fully
marine cetaceans are known from the fossil record. How was the
gap between a walking mammal and a swimming whale bridged?
Missing until recently were fossils clearly intermediate, or transi-
tional, between land mammals and cetaceans.
Very exciting discoveries have finally allowed scientists to re-
construct the most likely origins of cetaceans. It all started in 1979
when a team looking for fossils in North Pakistan found what
proved to be the oldest known fossil whale. The fossil was officially
described as Pakicetus in honor of the country where the discovery
was made. Pakicetus was found embedded in rocks formed from
river deposits that were 52 million years old. The river was actually
not far from the shores of the former Tethys Sea (see “Continental
Drift and the Changing Oceans,” p. 33).
The fossil consists of a complete skull of an archeocyte, an ex-
tinct group of ancestors of modern cetaceans. Though limited to a
skull, the Pakicetus fossil provides precious details on the origin of
cetaceans. The skull is cetacean-like but its jawbones lack the en-
larged space that is filled with fat or oil and used for receiving under-
water sound in modern whales (see “Echolocation,” p. 201). Pakicetus
probably detected sound through the ear opening as in land mam-
mals. The skull also lacks a blowhole, another adaptation for diving
in cetaceans. Other features, however, show experts that Pakicetus is a
transitional form between a group of extinct flesh-eating mammals,
the mesonychids, and cetaceans. It has been suggested that Pakicetus
fed on fish in shallow water and was not yet adapted for life in the
open ocean. It probably bred and gave birth on land.
Another major discovery was made in Egypt in 1989. Several
skeletons of another early whale, Basilosaurus, were found in sediments
left by the Tethys Sea and now exposed in the Sahara Desert. This
whale lived around 40 million years ago, 12 million years after Pakice-
tus. Many incomplete skeletons were found but they included, for the
first time in an archeocyte, a complete hind leg that features feet with
three tiny toes! The legs are small, far too small to have supported the
50-foot long Basilosaurus on land. Basilosaurus was undoubtedly a fully
marine whale with possibly non-functional, or vestigial, hind legs.
Another remarkable find was reported in 1994, also from
Pakistan. The now extinct whale, Ambulocetus natans (“the walking
whale that swam”), lived in the Tethys Sea 49 millon years ago. It
lived around 3 millon years after Pakicetus but 9 million years be-
fore Basilosaurus. The fossil luckily includes a good portion of the
hind legs. The legs were strong and ended in long feet very much
like those of a modern pinniped. The legs were certainly func-
tional both on land and sea. The whale still retained a tail and
lacked a fluke, the major means of locomotion in modern
cetaceans. The structure of the backbone shows, however, that
Ambulocetus swam like modern whales by moving the rear portion
of its body up and down, even if a fluke was missing. The large
hind legs were used for propulsion in water. On land, where it
probably bred and gave birth, Ambulocetus may have moved around
very much like a sea lion. It was undoubtedly a whale that linked
life on land with life at sea.
Even more exciting are recent findings in Pakistan, reported in
2001, of yet other fossil skeletons. These fossils link early cetaceans
with ungulates, the group that includes animals such as cattle,
sheep, pigs, and hippos. Some of the oldest bones (at least 50 mil-
lion years old) were from land-living, wolf-like, hoofed animals.
The finding of these fossil whales is one of the most exciting recent
discoveries in marine biology. Once an important discovery is made,
it is only the beginning. It spurs new interest and nearly always leads
to more new questions than answers.
THE WHALES THAT WALKED TO SEA
Ambulocetus natans, the walking whale that
swam.
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Chapter 9 Marine Reptiles, Birds, and Mammals 193
Baleen plate Mouth open, water in Mouth closed, water out
Baleen Upper jawbone
Upper jawbone
Lower
jawbone
Lower
jawbone
Baleen plate
Baleen
Baleen
Tongue
Right whale
Blue whale
Tongue
contracted
Tongue
contracted
Tongue
raised
Tongue
raised
FIGURE 9.17 The filtering apparatus of whales consists of vertical baleen plates. The number and length of plates vary in different species, up to
an average of 360 on each side in the sei whale (Balaenoptera borealis). The plates vary from 30 cm (1 ft) long in the minke whale (B. acutorostrata) to 
4.5 m (15 ft) in the bowhead whale (B. mysticetus). The baleen, also called whalebone, was once used to make corset stays, backings for billiard tables,
and buggy whips. Water is filtered as the mouthcloses and the tongue (yellow arrow) pushes up, forcing the water out through the baleen.
Right whale
Males and females—15 m (50 ft)
Bottlenose dolphin
Males and females—3 m (10 ft)
BALEEN
WHALES
Common porpoise
Males and females—1.4 m (5 ft)
Fin whale
Male—21 m (69 ft)
Female—22 m (72 ft) Killer whaleMale—8 m (26 ft)
Female—7 m (23 ft)TOOTHEDWHALES
Pilot whale
Male—5.5 m (18 ft)
Female—4.3 m (14 ft)
Sperm whale
Male—15 m (50 ft)
Female—11 m (36 ft)
Blue whale
Male—25 m (83 ft)
Female—26 m (86 ft)
Gray whale
Male—12 m (40 ft)
Female—13 m (43 ft)
FIGURE 9.18 Representative baleen and toothed whales.
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(B. acutorostrata)—together with two other
related species—are known as the rorquals.
They and the humpback whale (Megaptera
novaeangliae; see photo on page 179),
which is often included among the rorquals,
feed by gulping up schools of fish and
swarms of krill. The lower part of the
throat expands when feeding; hence the
distinctive accordion-like grooves on the
underside of these whales. Humpback
whales often herd fish by blowing curtains
of bubbles around them. Krill is the most
important part of the rorqual diet, especially
in the Southern Hemisphere, but fishes
such as herring and mackerel are also eaten
(Table 9.1).
The right whales (Eubalaena, Caper-
aea) and the bowhead whale (Balaena
mysticetus) feed by swimming along the
surface with their huge mouths open
(Fig. 9.18). They have the largest baleen
plates of the whales but the finest bristles
(Fig. 9.17). This allows them to filter
small plankton like copepods and some
krill (Table 9.1).
Gray whales (Eschrichtius robustus)
are primarily bottom feeders. When ex-
amined, their stomachs contain mostly
amphipods that inhabit soft bottoms
(Table 9.1). Grays stir up the bottom
194 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
Bottom Large Small Large Small Mesopelagic Miscellaneous
Whale Invertebrates Zooplankton Squids Squids Pelagic Fishes* Fishes
Species (%) (%) (%) (%) Fishes (%) (%) (%)
Blue — 100 — — — — —
Bowhead 20 80 — — — — —
Bryde’s — 40 — — 20 20 20
Fin — 80 5 — 5 5 5
Gray 90 5 — — — 5 —
Humpback — 55 — — 15 — 30
Minke — 65 — — 30 — 5
Northern Right — 100 — — — — —
Southern Right — 100 — — — — —
Sei — 80 5 — 5 5 5
Sperm 5 — 10 60 5 5 15
*Mesopelagic fishes are those found at depths of around 200 to 1,000 m (660 to 3,300 ft). 
Source: Adapted from D. Pauly, et al., 1998, ICES Journal of Marine Science, 55:467–481.
Diet of Great WhalesTable 9.1Table 9.1
with their pointed snouts and then filter
the sediment (Fig. 9.18), leaving charac-
teristic pits on the bottom. Most appear
to feed on their right sides because the
baleen on this side is more worn. Some,
however, are “left-handed” and feed on
the left side. A 10-week-old female kept
in captivity in San Diego, California, ate
over 815 kg (1,800 lb) of squid every day,
gaining weight at the rate of 1 kg (2.2 lb)
an hour!
The roughly 80 remaining species of
cetaceans are toothed whales that lack
baleen. Their teeth are adapted for a diet
of fish, squid, and other prey. They use
the teeth only to catch and hold prey, not
to chew it. Food is swallowed whole. As
in all cetaceans, food is ground up in one
of the three compartments of the stom-
ach. The blowhole has one opening, as
opposed to two in the baleen whales.
•
The toothed whales, which include the dolphins
and porpoises, lack baleen and feed on fish,
squid, and other prey.
•
The largest toothed whale is the
sperm whale (Physeter catodon), the un-
mistakable blunt-nosed giant of Moby
Dick fame (Fig. 9.18). Together, the
sperm and baleen whales are often called
the great whales. There is growing evi-
dence that sperm whales, though toothed,
are more closely related to baleen whales
than to other toothed whales. The sperm
whale is now the most numerous of the
great whales, even though it was the
mainstay of the whaling industry for cen-
turies (see Table 9.2, p. 196). The largest
on record weighed 38,000 kg (42 tons).
Sperm whales are fond of squid, in-
cluding the giant deep-sea ones. Undi-
gested squid beaks and other debris
accumulate in the gut as large globs of
sticky material known as ambergris. Be-
lieve it or not, ambergris is an ingredient
in fine perfumes. Sperm whales also eat a
wide variety of fishes (including sharks),
lobsters, and other marine animals
(Table 9.1).
The other toothed whales are much
smaller than the great whales. One is the
killer whale, or orca (Orcinus orca; see
Figs. 9.18 and 10.7), a magnificent black
and white predator with a taste for seals,
penguins, fishes, sea otters, and even
other whales. They use their flukes to
stun their prey when feeding on schools
of herring—more efficient than chasing
individual fish. Killer whales are most
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common in cold water but are found
around the world. A few are kept in cap-
tivity. Killer whales have a nasty reputa-
tion, but there are no confirmed cases of
them attacking humans in the wild.
Though they are all whales, most of
the small toothed whales are called dol-
phins or porpoises. Technically, porpoises
comprise only a small group of blunt-nosed
whales (Fig. 9.18), but in some places the
name “porpoise” is given to some of the
dolphins. Most of the small whales, how-
ever, are called dolphins and some people
prefer to call all of them dolphins.
The many species of dolphins typi-
cally possess a distinctive snout, or beak,
and a perpetual “smile.” Playful, highly so-
cial, and easily trained, dolphins easily win
people’s hearts. They often travel in large
groups called pods, herds, or schools.
They like to catch rides along the bows of
boats (Fig. 9.19a) or even around great
whales. The bottlenose dolphin (Tursiops
truncatus) is the dolphin seen in marine
parks and oceanaria around the world.
The spinner dolphin (Stenella longirostris;
Fig. 9.19b) is so named because of its
spectacular twisting jumps in the air. It is
one of the species of dolphins that get
caught in the nets of tuna fishers. This
happens because the tuna and dolphins eat
the same fish and often occur together.
Dolphins are not the only cetaceans
to be threatened. Whale hunting, or
whaling, is an old tradition with a rich
history. Native Americans hunted gray
whales in prehistoric times; Eskimos still
legally hunt them. Basques may have
hunted them off Newfoundland before
Columbus. It was not until the 1600s,
however, that Europeans started to sub-
stantially exploit the great whales in the
North Atlantic. Americans, who eventu-
ally dominated worldwide whaling, began
hunting off New England by the late
1600s. Whales were harpooned from
small open boats (Fig. 9.20), a technique
whalers learned from the natives. It was a
rewarding fishery, though not one ex-
ploited primarily for food. Blubber pro-
vided “train oil” that was used to make
soap and as lamp oil. Baleen was used to
make stays for corsets and other goods.
Meat and other valuable products also
were obtained from the huge animals.
Whaling efforts rapidly increased after
fast steamships and the devastating ex-
plosive harpoon were introduced in the
1800s. The largest and fastest whales, like
the blue whale and the fin whale, were
then at the mercy of whalers.
Chapter 9 Marine Reptiles, Birds, and Mammals 195
(a)
(b)
FIGURE 9.19 Dolphins often ride the
bow wave of boats (a) or even that of whales.
They ride without beating their tails, obtaining
thrust from the pressure wavein front of the
ship. (b) This spinner dolphin (Stenella
longirostris) from the Eastern Pacific was
photographed swimming alongside a ship.
Amphipods Small crustaceans whose
bodies are compressed from side to side.
Chapter 7, p. 136; Figure 7.30
FIGURE 9.20 Sperm whales being harpooned in the South Pacific by the crew of the
Acushnet, an American whale ship from Fairhaven, Massachusetts. This watercolor painting is part
of the sea journal of an 1845 to 1847 voyage. Herman Melville, the author of Moby Dick, sailed as a
seaman on the Acushnet from 1841 to 1842.
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Whales are long-lived mammals with
a very low reproductive rate. The great
whales generally give birth to one well-
developed calf that has been carried by the
mother for a year or more (see “Biology of
Marine Mammals,” p. 198). Females usu-
ally don’t become pregnant for one or two
years after giving birth. As a result of this
low reproductive potential, whale stocks
could not stand the intense whaling pres-
sure, and many of the fisheries collapsed.
Almost all great whales are now classified
as endangered (Table 9.2).
The first to be seriously depleted was
the slow-swimming North Atlantic right
whale (Eubalaena glacialis), the “right”
species to be killed, according to whalers,
because it floated after being harpooned.
By the early 1900s whaling had moved to
the rich feeding grounds around Antarc-
tica. This location proved to be a real bo-
nanza. Whaling nations developed
factory ships able to process whole car-
casses. The Antarctic fishery reached its
peak in the 1930s. The whales received a
reprieve during World War II, but it was
too late for saving the fisheries. It is esti-
mated that more than a million whales
were taken from Antarctica alone.
Blue whales, the largest of them all,
were especially sought. A large specimen
yielded more than 9,000 gallons of oil. It
has been estimated that over 200,000 blue
whales were taken worldwide between
1924 and 1971, close to 30,000 during the
1930–31 whaling season alone. Catches
climbed way above the optimal yield level.
Catch per whaler-day’s effort declined
every year after 1936. As many as 80% of
all blue whales caught by 1963 were sexu-
ally immature, so that there were even
fewer individuals in the ocean able to per-
petuate the species.
Fin whales, the second largest of all
whales, became the next major target as
blue whales became more and more
scarce. The 1950s and early 1960s saw
annual catches of 20,000 to 32,000 fin
whales per year, mostly from Antarctica.
As their stocks dwindled, whalers shifted
their target again in the mid-1960s, this
time to the smaller sei whale (Balaeno-
ptera borealis). The sei whale averages a
length of around 13 m (44 ft), whereas
the fin whale averages 20 m (65 ft).
•
Intense whaling has led to the near extermina-
tion of most species of great whales. Practically
all of these species are now endangered.
•
The abrupt disappearance of the
more commercially valuable whales, one
after the other, meant lower profits for the
whaling industry. In 1946, 20 whaling na-
tions established the International Whal-
ing Commission (IWC) in an attempt to
regulate whale hunting to stop overfish-
ing. It collected data on the number of
whales, though the numbers came mostly
from the whalers themselves. It set annual
quotas for the number of whales to be
killed each year, quotas that unfortunately
were non-binding and could not be en-
forced. Furthermore, some whaling na-
tions did not belong to the IWC. Saving
the whaling industry was considered more
important than saving the whales. The
blue whale was not completely protected
by the IWC until the 1965–66 season,
long after its numbers had been drastically
reduced; by then blue whales were so hard
to find that the fishery for them was no
longer a profitable fishery. Even under the
protection of the IWC, blue whales were
hunted at least until 1971 by the fleets of
countries that did not belong to the IWC.
Under mounting pressure from con-
servationists, the IWC gradually banned
the hunting of other whales. Demand for
whale products, mostly oil used in the
manufacture of margarine and lubricants,
was reduced because substitutes had been
found for most of them. Whale meat,
however, continued to be used as pet
food and is still valued as human food,
mostly in Japan. The lower quotas of the
196 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
Whale Species Status Estimated Pre-Exploitation Number Estimated Number in the Late 1990s
Blue Endangered 160,000–240,000 5,000
Bowhead Endangered 52,000–60,000 8,200
Bryde’s Protected 100,000 66,000–86,000
Fin Endangered 300,000–650,000 123,000
Gray (eastern Pacific) Protected 15,000–20,000 26,000
Gray (western Pacific) Endangered 1,500–10,000 100–200
Gray (Atlantic) Extinct Unknown 0
Humpback Endangered 150,000 25,000
Minke Hunted 350,000 850,000
Northern right Endangered Unknown 870–1,700
Southern right Endangered 100,000 1,500
Sei Endangered 100,000 55,000
Sperm Endangered >2,000,000 >1,000,000
Source: International Whaling Commission (IWC) and others.
Estimated Numbers of Great Whales before Exploitation and during the Late 1990sTable 9.2Table 9.2
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IWC were unfortunately not always ac-
cepted by all nations.
The United States Congress sepa-
rately passed the Marine Mammal Pro-
tection Act of 1972, which bans the
hunting of all marine mammals in U.S.
waters (except in the traditional fisheries
of Alaskan natives; see Fig. 4.2) and the
importation of marine mammal prod-
ucts. By 1974 the IWC had protected
the blue, gray, humpback, and right
whales around the world, but only after
their stocks were no longer economically
viable. Sperm, minke, fin, and sei whales
were still hunted in large numbers, but
worldwide catches began to dwindle.
Catches of these whales fell from 64,418
in 1965 to 38,892 in 1975 and to 6,623
in 1985. A moratorium on all commer-
cial whaling was finally declared by the
IWC in 1985, a move long sought by
conservationist groups. The former So-
viet Union halted all whaling in 1987.
Japan, Iceland, and Norway, however,
opted in 1988 to continue hunting
minke, fin, and sei whales, as allowed by
the IWC under the controversial title of
“scientific whaling.” Iceland eventually
quit the IWC.
In 1994 IWC members signed an
agreement that created a vast sanctuary
for all whales in the waters around
Antarctica. This area is the main feeding
ground for 80% of the surviving great
whales. Japan voted against the agree-
ment and independently decided to con-
tinue hunting whales in Antarctica.
Starting with the 1997–98 season Japan
took 440 minkes from Antarctica and
100 from the North Pacific each season.
The 2002 season saw Japan expand its
North Pacific catch to include 150
minke, 50 Bryde’s (Balaenoptera edeni),
10 sperm, and for the first time since
1987, 50 sei whales. In 1997 Norway an-
nounced its resumption of commercial
whaling of minkes in the North Sea in
defiance of the IWC (Fig. 9.21). Norway
allowed the killing of 671 whales during
the 1998–99 season. Meanwhile, hunting
of smaller cetaceans not protected by the
IWC, like the Dall porpoise (Phocoenoides
dalli), has increased in an attempt to find
substitutes for whale meat.
Nobody knows when the great
whales will again roam the oceans in
numbers approaching those before the
start of large-scale whaling. Some experts
are afraid that a few critically endangered
species will never recover completely.
Small-scalewhaling remains part of the
traditional fisheries of the native inhabi-
tants of the Arctic region from Greenland
to Siberia and in the Lesser Antilles in the
Caribbean. One of the whales hunted in
the Arctic, the bowhead, and another in
the Lesser Antilles, the humpback, are
endangered. Other smaller whales—the
killer whale, narwhal (Monodon monoc-
eros), and beluga (Delphinapterus leucas)—
are also hunted in the Arctic.
Recovery is under way in other
species. The California gray whale, pro-
tected since 1947, has made a phenomenal
comeback (Fig. 9.22). It was removed
from the endangered species list in 1994.
In 1997 the IWC allowed the killing of
600 gray whales by native hunters in
Siberia and 20 by the Makah Indian tribe
in the state of Washington. Only one,
however, was killed in 1999. Even the blue
whale, whose reproduction is severely lim-
ited by its restriction to small populations
scattered around the world, is making a
comeback of sorts. It has returned to the
southern reaches of the Arctic Ocean
north of Norway, a region where they
flourished before their near extermination
by whalers. Sightings in California waters
have increased sharply. Their numbers in
Antarctica, however, are even lower than
first estimates: around 500 animals, or
only 0.2% of those feeding there before
whaling began.
Dolphins, not protected by the IWC,
are also at great risk (see Table 18.1,
p. 422). They have replaced the larger
whales as the most threatened of all
cetaceans. As many as 28 species of small
cetaceans are in immediate danger of ex-
tinction. Only 200 to 500 vaquitas, or “lit-
tle cows” (Phocoena sinus), are left. This
shy, shovel-nosed porpoise, known only
from the northern Gulf of California, re-
mained unknown to science until 1958.
Everywhere, fishers are depleting stocks
of fish and squid on which dolphins feed.
Dolphins themselves are being hunted for
human food. It is becoming popular in
countries like Perú, where dolphin meat is
cheaper than beef or chicken.
Chapter 9 Marine Reptiles, Birds, and Mammals 197
FIGURE 9.21 A harpooned minke
whale (Balaenoptera acutorostrata) being
hauled on board a Norwegian whaling ship in
the North Sea.
FIGURE 9.22 California gray whales
(Eschrichtius robustus) are once again a
common sight along their long migration
routes from Alaska to Mexico (see Fig. 9.31).
This species was removed from the
endangered species list in 1994.
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Tuna fishers using giant purse seine
nets (see Fig. 17.6b) trap and drown the
many dolphins that often swim above
schools of tuna, mostly yellowfin tuna
(Thunnus albacares) in the Eastern Pa-
cific. Fishers often find their catch after
spotting dolphins, which is known as
“setting on tuna.” During the early 1970s
an estimated 200,000 dolphins died an-
nually, mostly in the hands of American
fishing fleets. The slaughter induced such
public outrage that the United States,
through the Marine Mammal Protection
Act of 1972, called for a reduction in the
accidental deaths of dolphins. It imposed
a quota of 20,500 for the number of dol-
phins that could be killed by American
fleets. The use of special nets was en-
forced, and observers were placed on
board vessels to verify compliance with
the ruling. By 1990 it was estimated that
the number of dolphins killed by the
United States tuna fleet, by then operat-
ing in “dolphin-safe” western Pacific wa-
ters, had reached zero. Environmentalists
won a major victory when in 1990 the
three biggest tuna packers in the United
States pledged not to buy or sell fish that
was caught using methods that injure or
kill dolphins. Tuna cans began to display
“dolphin-safe” labels, and imported tuna
caught without the use of dolphin-safe
methods were banned from sale in the
United States. In 1997 the ban was lifted
from some countries, notably Mexico,
where fishers improved their methods.
Dolphins, however, continue to
drown in purse seine nets, mostly in the
tropical Eastern Pacific at the hands of
unregulated fishing fleets. The number of
dolphins in the Eastern Pacific has no-
ticeably decreased, particularly among the
coastal spotted (Stenella attenuata) and
eastern spinner (see Fig. 9.19b) dolphins.
Dolphins have also been entangled
and killed by the thousands in huge drift
nets (see Fig. 17.6e), which also threaten
sharks, sea turtles, seals, seabirds, and
other marine life (Fig. 9.23). The nets, in
some cases as large as 60 km (37 mi)
long and 15 m (50 ft) deep, have been
used to catch fish and squid, but they ac-
tually catch practically anything that tries
to swim by. Not only do they deplete
valuable commercial fisheries like alba-
core tuna and salmon, but they trap
many noncommercial species. These
“walls of death” also are very wasteful be-
cause a large percentage of the catch
drops out during hauling. Their use in
the North Pacific salmon fishery has
been particularly deadly to the Dall por-
poise. Hundreds of fishing boats outfit-
ted for drift netting have been used to
catch tuna in the South Pacific with po-
tentially disastrous results. International
pressure persuaded Japan, which had the
largest fleet of drift-net boats in the Pa-
cific, and Taiwan to end the use of drift-
net fishing in 1993.
Biology of Marine
Mammals
It is surprising how little we know about
marine mammals. Most are difficult or
impossible to keep in captivity or even to
observe for long periods at sea. Some
whales and dolphins are rarely seen, so
what little we know about them comes
from captive or stranded individuals and
information gathered over the years by
whalers. What we do know about marine
mammals, however, is simply fascinating.
198 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
FIGURE 9.23 This Pacific white-sided dolphin (Lagenorhynchus obliquidens) drowned after
getting caught in a drift net in the North Pacific.
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Swimming and Diving
Streamlining of the body for swimming is
a hallmark of marine mammals. Seals, sea
lions, and other pinnipeds swim mostly
by paddling with their flippers. Sirenians
and cetaceans, in contrast, move their
tails and flukes up and down (Fig. 9.24).
Fishes, you will recall, move their tails
from side to side (see Fig. 8.11). Ce-
taceans turn mostly by up-and-down
movements of the tail and flukes. Sea
lions have been timed at speeds of 35 kph
(22 mph). Blue and killer whales can
reach speeds of 50 kph (30 mph). A
group of common dolphins (Delphinus
delphis) was recorded bowriding at a
speed of 64 kph (40 mph)!
Cetaceans have the advantage of hav-
ing the blowhole on top of the head. This
allows them to breathe even though most
of the body is underwater. It also means,
by the way, that cetaceans can eat and
swallow without drowning. To avoid in-
haling water, marine mammals take very
quick breaths. A fin whale can empty and
refill its lungs in less than 2 seconds, half
the time we take, even though the whale
breathes in 3,000 times more air! When
swimming fast, many pinnipeds and dol-
phins jump clear out of the water to take a
breath.
In the large whales the moisture in
their warm breath condenses when it hits
the air. Together with a little mucus and
seawater, this water vapor forms the char-
acteristic spout, or blow (see Fig. 1.20).
The spout can be seen at great distances
and its height and angle used to identify
the whale (Fig. 9.25). The blue whale, for
instance, has a spout some 6 to 12 m (20 to
40 ft) high.
To keep warm in cold water, the
great whales depend on a thick layer of
blubber(see Fig. 4.2). Feeding, however,
leaves their huge mouths exposed to low
temperatures, a major problem in the very
cold polar waters where they normally
Chapter 9 Marine Reptiles, Birds, and Mammals 199
FIGURE 9.24 Swimming in cetaceans involves strong up-and-down movements of the tail and flukes.
Blue whale
Surfacing and blowing Start of dive End of dive
Fin whale
Gray whale
Right whale
Sperm whale
Humpback
 whale
FIGURE 9.25 Great whales can be identified from a distance by their blowing pattern, their outline on the surface, and the way they dive.
Castro−Huber: Marine 
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feed. It has been recently discovered that
a network of blood vessels in their
tongues actually reduces heat loss by
transferring heat from warm blood into
vessels that carry it back to the body core.
Marine mammals have mastered the
art of diving, and most make prolonged
dives to considerable depths for food.
There is a wide range in diving ability. Sea
otters can dive for only 4 or 5 minutes, to
depths of perhaps 55 m (180 ft). Pin-
nipeds normally dive for up to 30 minutes,
and maximum depths are roughly 150 to
250 m (490 to 820 ft). Female northern
elephant seals (Mirounga angustirostris),
however, are capable of continuous deep
dives of up to 400 m (1,300 ft). One indi-
vidual was recorded diving to a depth of
1,500 m (5,000 ft). The Weddell seal
(Leptonychotes weddelli) has been recorded
diving for as long as 1 hour 13 minutes
and as deep as 575 m (1,900 ft).
The plankton-feeding habits of
baleen whales do not require them to dive
too deeply for their food, and they sel-
dom venture below 100 m (300 ft).
Toothed whales, however, are excellent
divers. Dolphins are known to dive as
deep as 300 m (990 ft). The champion
diver is the sperm whale, which can stay
under for at least an hour. They are
known to dive to 2,250 m (7,380 ft) and
can probably go much deeper.
The long, deep dives of marine
mammals require several crucial adapta-
tions. For one thing, they must be able to
go a long time without breathing. This
involves more than just holding their
breath, for they must keep their vital or-
gans supplied with oxygen. To get as
much oxygen as possible before dives,
pinnipeds and cetaceans hold their breath
for 15 to 30 seconds, then rapidly exhale
and take a new breath. As much as 90%
of the oxygen contained in the lungs is
exchanged during each breath, in contrast
to 20% in humans.
Not only do diving marine mammals
breathe more air faster than other mam-
mals, they are better at absorbing the
oxygen from the air and storing it in their
blood. They have relatively more blood
than non-diving mammals. Their blood
also contains a higher concentration of
erythrocytes, or red blood cells, and these
cells carry more hemoglobin. Further-
more, their muscles are extra rich in myo-
globin, which means that the muscles
themselves can store a lot of oxygen.
Marine mammals have adaptations
that reduce oxygen consumption in addi-
tion to increasing supply. When they dive,
the heart rate slows dramatically. In the
northern elephant seal, for example, the
heart rate decreases from about 85 beats
per minute to about 12. Blood flow to
non-essential parts of the body, like the
extremities and the intestine, is reduced,
but it is maintained to vital organs like the
brain and heart. Thus, oxygen is made
available where it is needed most when
oxygen supply is cut off during a dive.
Another potential problem faced by
air-breathing, diving animals (including
human divers) results from the presence
of large amounts of nitrogen (70% of
total volume) in the air. Nitrogen dis-
solves much better at high pressures, like
those experienced at depth. The blood of
scuba divers picks up nitrogen while they
are below the surface. If the pressure is
suddenly released, some of the nitrogen
will not stay dissolved and will form tiny
bubbles in the bloodstream. You can see a
200 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
It is widely known that toothed whales use sound for echolocation
and to communicate with each other. Recently a different use of
sound waves by these cetaceans has been suggested.
This new hypothesis developed as a possible explanation for the
feeding habits of sperm whales, the largest of the toothed whales.
Squids taken from the stomach of captured and stranded whales
often show no tooth marks or scars of any kind. In fact, live squids
have been known to swim out of the stomachs of freshly caught
whales! It seems that sperm whales have a way of catching squids—
including giant squids—without using their teeth, even if their teeth
are actually of little help because they are present only in the lower
jaw. Another puzzle is explaining how sperm whales weighing
36,000 kg (40 tons) or more and averaging speeds of just 2 to 4
knots catch squids that can swim at 30 knots.
How about the possibility that whales and dolphins may use
powerful blasts of sound to catch their food? This ingenious hy-
pothesis has been dubbed a second “big bang theory,” the original
being the well-known view proposed to explain the origin of our
universe (see “The Structure of the Earth,” p. 22). Catching prey
whole and still alive could be explained if the whale stuns its prey
with a blast of sound and then simply swallows it whole.
Some indirect evidence is provided by the now-extinct ances-
tors of toothed whales. Fossils of the earliest known toothed whales
have long snouts armed with many piercing teeth (see “The Whales
That Walked to Sea,” p. 192). Like those of a barracuda, the teeth
were probably used to catch small fish and other prey. The long
snout, however, has disappeared in most modern cetaceans, and the
teeth have become wider and shorter. Have modern toothed whales
evolved a new technique to catch their food, or has their food source
changed?
Sonic hunting may involve a beam of low-frequency sound
waves powerful enough to stun a fish or squid. Although the sophis-
ticated sound-producing mechanism of cetaceans is not fully under-
stood, it is thought to be capable of emitting the required sound
waves. It has been suggested that sonic hunting evolved as a by-
product of echolocation in the early toothed whales.
It is not easy to obtain the necessary evidence to support this
hypothesis. Loud noises can indeed stun fish, but it is difficult to re-
produce the exact sounds produced by cetaceans. Dolphins living in
the wild have produced gun-like bangs that can be heard by hu-
mans. Unfortunately, undertaking detailed studies of sonic hunting
in the wild presents many complications. Captive dolphins do not
produce loud noises, which is not surprising because the echo of the
noise off the tank walls would be very painful to them. The function
of big bangs in whales is but one of the many surprising adaptations
of cetaceans waiting to be explored.
THE OTHER “BIG BANG THEORY”
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similar phenomenon when you open a
bottle of soda pop. As long as the top is
on, the contents are under pressure. The
carbonation, actually carbon dioxide gas,
remains dissolved. When you open the
bottle the pressure is released and bubbles
form. When nitrogen bubbles form in 
the blood after diving, they can lodge in
the joints or block the flow of blood to
the brain and other organs. This pro-
duces a horribly painful condition known
as the bends. To avoid the bends, human
divers must be very careful about how
deep they go, how long they stay under-
water, and how fast they come up.
Marine mammals dive deeper and
stay downlonger than human divers, so
why don’t they get the bends? The an-
swer is that they have adaptations that
prevent nitrogen from dissolving in the
blood in the first place. Human lungs
work pretty much the same while scuba
diving underwater as on land. When ma-
rine mammals dive, on the other hand,
their lungs actually collapse. They have a
flexible rib cage that gets pushed in by
the pressure of the water. This squeezes
the air out of peripheral areas of the
lungs where it readily dissolves into the
blood. Air is moved instead into the 
central spaces of the lungs, where little
nitrogen is absorbed. Some pinnipeds ac-
tually exhale before they dive, further re-
ducing the amount of air—and therefore
nitrogen—in the lungs.
•
Adaptations for deep, prolonged dives in ma-
rine mammals include efficient exchange of air
on the surface, storage of more oxygen in the
blood and muscles, reduction of the blood supply
to the extremities, and collapsible lungs to pre-
vent the bends.
•
Echolocation
Marine mammals depend little on the
sense of smell, which is so important to
their terrestrial cousins. Their vision is
excellent, but they have developed an-
other sensory system, echolocation,
based on hearing. Echolocation is na-
ture’s version of sonar. Most if not all
toothed whales, including dolphins and
porpoises, and some pinnipeds are known
to echolocate. At least some baleen
whales may use echolocation. Echoloca-
tion is not exclusive to marine mammals.
Bats, for example, use it to find insects
and other prey while flying at night.
Toothed whales echolocate by emit-
ting sound waves, which travel about five
times faster in water than in air, and lis-
ten for the echoes that are reflected back
from surrounding objects (Fig. 9.26).
The echoes are then analyzed by the
brain. The time it takes the echoes to re-
turn tells the animals how far away the
object is.
•
Many marine mammals echolocate by analyz-
ing the echo of sound waves they emit. Echolo-
cation is used to find prey and orient to the
surroundings.
•
The sounds used in echolocation
consist of short bursts of sharp clicks that
are repeated at different frequencies.
Low-frequency clicks have a high pen-
etrating power and can travel long dis-
tances. They reflect from large features
and are used to obtain information on the
surrounding topography. Low-frequency
sound waves may also be used in some
toothed whales to stun their prey (see
“The Other ‘Big Bang Theory,’ ” p. 200).
To discriminate more detail and locate
nearby prey, high-frequency clicks that
Chapter 9 Marine Reptiles, Birds, and Mammals 201
Hemoglobin A blood protein that
transports oxygen in many animals; 
in vertebrates it is contained in
erythrocytes, or red blood cells.
Myoglobin A muscle protein in many
animals that stores oxygen.
Chapter 8, p. 166
Brain
Skull
Blowhole
Air sac
Melon
Nasal plug
Echolocation clicks
Echo of clicks
Target
Acoustic window
Inner ear
Air passages to lungs
FIGURE 9.26 Dolphins echolocate by emitting bursts of sound waves, or clicks, by pushing air through internal air passages. Two muscular
nasal plugs act as valves, closing and opening the passages. Flaps of tissue on the plugs probably also produce sound by vibrating in the moving air. The
clicks are focused into a beam by the melon. To cover a wider area the dolphin moves its head from side to side. The melon is also known to receive the
echoes and transmit them to the ears, but most are received by the lower jaw.
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202 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
(a)
(b)
FIGURE 9.27 (a) The beluga (Delphinapterus leucas) is a white Arctic whale (beluga means “white one” in Russian) with a conspicuous melon. 
(b) In their natural environment belugas live in small groups.
are inaudible to humans are used. Ex-
periments have shown that blindfolded 
bottlenose dolphins can discriminate be-
tween objects of slightly different size or
made of different types of materials and
even detect wires.
We are not completely sure how
echolocation operates in marine mam-
mals. The process is rather complicated
(Fig. 9.26). The clicks, squeaks, and
whistles of cetaceans are produced as air
is forced through the air passages and
several associated air sacs while the blow-
hole is closed. The frequency of the
clicks is changed, or modulated, by con-
tracting and relaxing muscles along the
air passages and sacs. A fatty structure on
the forehead of toothed whales, the
melon, appears to focus and direct the
sound waves. The melon gives these
whales their characteristic rounded fore-
heads. To accommodate the melon, the
skull is modified to form a pointed, dish-
shaped face. The skull is also asymmet-
ric, the right side being slightly different
from the left side. Belugas (Fig. 9.27)
have a bulging forehead that changes
shape as the melon, moved by muscles,
focuses the sound. The huge forehead of
the sperm whale is filled in part with a
massive melon called the spermaceti
organ. Whalers originally thought this
was the sperm sac of the whale, hence
the peculiar name. This organ is filled
with a waxy oil, spermaceti, once much
sought for making candles and still used
as a lubricant for precision instruments.
The actual function of the spermaceti
organ is a controversial issue. It has also
been suggested that the deep-diving
sperm whale might also use the sperma-
ceti organ to regulate buoyancy or to ab-
sorb excess nitrogen.
In toothed whales incoming sound
waves are received primarily by the lower
jaw (Fig. 9.26). The ear canal that con-
nects the outside with the inner ear is re-
duced or blocked in most cetaceans. The
jawbones, filled with fat or oil, transmit
sound to the two very sensitive inner ears.
Each ear receives sound independently.
The ears are protected by a bony case and
embedded in an oily mixture that insu-
lates the ear but allows sound waves to
pass from the jaws. Sound information is
sent to the brain, which forms a mental
“picture” of the target or surroundings. 
In fact, sight and sound information
seem to be handled similarly by the brain.
Captive dolphins can recognize by echo-
location objects they have seen and rec-
ognize by sight those they have previously
echolocated.
Behavior
Echolocation is just one indication of the
amazing mental capabilities of marine
mammals. The mammalian brain has
evolved as an association center for com-
plex behaviors in which learning, not in-
stinct, dominates. In contrast to fishes,
birds, and other vertebrates, mammals
rely mostly on past experience, stored and
processed by the brain, to respond to
changes in the environment (see “How
Intelligent Are Cetaceans?,” p. 203).
Most marine mammals are highly so-
cial animals that live in groups at least part
of the time. Many pinnipeds live in huge
colonies during the breeding season. Most
cetaceans spend their entire lives in highly
organized pods of a few (Fig. 9.27b) to
thousands of individuals. Some pods in-
clude smaller subgroups organized by age
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and sex. To keep in contact, many of their
highly complex and sophisticated behav-
iors are directed toward members of their
own species.
Sounds, or vocalizations, play a
prominent role in communication. Sea
lions and fur seals communicate by loud
barks and whimpers; seals use more se-
date grunts, whistles, and chirps. The vo-
calizations of pinnipeds are especially
important in maintaining territories dur-
ing reproduction (see “Reproduction,”p. 207). Females and their pups or calves
recognize each other by their “voices.”
Cetaceans produce a rich variety of
vocalizations that are different from the
sounds used for echolocation. Both types
of sounds can be produced simultane-
ously, providing further evidence of the
complexity of sound production in ma-
rine mammals. Social vocalizations are
Chapter 9 Marine Reptiles, Birds, and Mammals 203
We often hear that whales, dolphins, and porpoises are as intelligent
as humans, maybe even more so. Are they really that smart? There
is no question that cetaceans are among the most intelligent of ani-
mals. Dolphins, killer whales, and pilot whales in captivity quickly
learn tricks. The military has trained bottlenose dolphins to find
bombs and missile heads and to work as underwater spies.
This type of learning, however, is called conditioning. The
animal simply learns that when it performs a particular behavior it
gets a reward, usually a fish. Many animals, including rats, birds,
and even invertebrates, can be conditioned to perform tricks. We
certainly don’t think of these animals as our mental rivals.
Unlike most other animals, however, dolphins quickly learn
by observation and may spontaneously imitate human activities.
One tame dolphin watched a diver cleaning an underwater view-
ing window, seized a feather in its beak, and began imitating the
diver—complete with sound effects! Dolphins have also been
seen imitating seals, turtles, and even water-skiers.
Given the seeming intelligence of cetaceans, people are al-
ways tempted to compare them with humans and other animals.
Studies on discrimination and problem-solving skills in the
bottlenose dolphin, for instance, have concluded that its intelli-
gence lies “somewhere between that of a dog and a chimpanzee.”
Such comparisons are unfair. It is important to realize that
intelligence is a very human concept and that we evaluate it in
human terms. After all, not many people would consider them-
selves stupid because they couldn’t locate and identify a fish by
its echo. Why should we judge cetaceans by their ability to solve
human problems?
Both humans and cetaceans have large brains with an ex-
panded and distinctively folded surface, the cortex. The cortex is
the dominant association center of the brain, where abilities such
as memory and sensory perception are centered. Cetaceans have
larger brains than ours, but the ratio of brain to body weight is
higher in humans. Again, direct comparisons are misleading. In
cetaceans it is mainly the portions of the brain associated with
hearing and the processing of sound information that are ex-
panded. The enlarged portions of our brain deal largely with vi-
sion and hand-eye coordination. Cetaceans and humans almost
certainly perceive the world in very different ways. Their world is
largely one of sounds, ours one of sights.
Contrary to what is depicted in movies and on television, the
notion of “talking” to dolphins is also misleading. Though they pro-
duce a rich repertoire
of complex sounds,
they lack vocal cords
and their brains prob-
ably process sound
differently from ours.
Bottlenose dolphins
have been trained to
make sounds through
the blowhole that
sound something like
human sounds, but
this is a far cry from
human speech. By the
same token, humans
cannot make whale sounds. We will probably never be able to carry
on an unaided conversation with cetaceans.
As in chimps, captive bottlenose dolphins have been taught
American Sign Language. These dolphins have learned to com-
municate with trainers who use sign language to ask simple ques-
tions. Dolphins answer back by pushing a “yes” or “no” paddle.
They have even been known to give spontaneous responses not
taught by the trainers. Evidence also indicates that these dolphins
can distinguish between commands that differ from each other
only by their word order, a truly remarkable achievement. Never-
theless, dolphins do not seem to have a real language like ours.
Unlike humans, dolphins probably cannot convey very complex
messages.
Observations of cetaceans in the wild have provided some
insights on their learning abilities. Several bottlenose dolphins
off Western Australia, for instance, have been observed carrying
large cone-shaped sponges over their beaks. They supposedly use
the sponges for protection against stingrays and other hazards on
the bottom as they search for fish to eat. This is the first record
of the use of tools among wild cetaceans.
Instead of “intelligence,” some people prefer to speak of
“awareness.” In any case, cetaceans probably have a very different
awareness and perception of their environment than do humans.
Maybe one day we will come to understand cetaceans on their
terms instead of ours, and perhaps we will discover a mental so-
phistication rivaling our own.
HOW INTELLIGENT A RE CETACEANS?
Bottlenose dolphins participating in research
to test acoustic communication. The devices
on their heads, which are held in place by
suction cups, light up every time a dolphin
whistles.
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low-frequency sounds that humans can
hear. The variety of sounds is amazing
and includes grunts, barks, squeaks,
chirps, and even “moos.” Different
sounds are associated with various moods
and are used in social and sexual signal-
ing. Whistles, emitted in a multitude of
variations and tones, are characteristic of
each species. Some of these sounds serve
as a “signature,” allowing individuals of
the same species to recognize one an-
other. Among the more than 70 calls that
have been identified among killer whales,
some are present in all individuals,
whereas others are “dialects” that identify
certain pods.
Sounds are also used to maintain the
distance between individuals and have an
important role in the structure of the
pod. Particular sounds are emitted during
breeding, feeding, alarms, and birth.
Mother gray whales grunt to stay in con-
tact with their calves. Fin whales make a
low-pitched sound thought to be in-
volved in long-distance communication.
Right whales have at least six distinct
calls, each related to a specific function.
The humpback whale is renowned
for its soulful songs. They are sung by
breeding males to attract females by ad-
vertising their readiness to mate. The
songs consist of phrases and themes re-
peated in a regular pattern for a half hour
or longer. They may be repeated over and
over for days! The songs change over
time. Males also start each breeding sea-
son with the song they were singing at
the end of the previous breeding season.
New songs learned from immigrants have
been shown to gain instant popularity
among native whales. Researchers record
and catalogue songs to help track whales
in their annual migrations.
Communication among cetaceans is
not restricted to vocalizations. Re-
searchers have described a variety of pos-
tures and movements that may indicate
the animal’s mood. Dolphins clap their
jaws or turn around with their mouths
open as a threat. The loud cracking
sound made when some marine mam-
mals flap their flukes or flippers on the
surface is thought to be a warning signal.
Cetaceans are noted for their play be-
havior, seemingly pleasurable activities
with no serious goal. Many species, in-
cluding the great whales and killer whales,
play with food or floating objects like
logs, kelp, and feathers, throwing them up
in the air or holding and pushing them
with their snouts. Individuals may swim
head down or on their backs apparently
just for the fun of it. Dolphins play with
rings of air bubbles they create. Dolphins
also like to surf, and pilot (Globicephala)
and right whales go sailing with their
flukes out of the waterto catch the wind.
Sex play, the rubbing and touching of the
genital opening, is also common.
The sight of a great whale breaching,
leaping up in the air and loudly crashing
on the surface, is awesome (Fig. 9.28).
Breaching has been variously interpreted
as a warning signal, as a way of scanning
the surface or the shoreline, as a means of
getting rid of external parasites or an ar-
dent lover, and simply as fun. After a deep
dive, sperm whales may breach, fall on
their backs, and make a splash that can 
be heard 4 km (2.5 mi) and seen 28 km
(17.4 mi) away! Many whales stick their
heads out of the water to spy on their sur-
roundings (Fig. 9.29a).
The complex behavior of cetaceans 
is evident in other ways. When one indi-
vidual is in trouble, others may come to
assist (Fig. 9.29b). Members of a pod re-
fuse to leave a wounded or dying com-
rade. Whalers knew that a harpooned
whale was a lure for others, who are
drawn from miles around. Dolphins will
carry injured individuals to the surface to
breathe (Fig. 9.29c), and there are records
of females carrying the body of a stillborn
calf until it rots.
Many toothed whales work together
when they hunt, some in coordinated
pairs. Sometimes whales take turns feed-
ing while their partners herd a school of
fish. An individual may investigate some-
thing strange lying ahead while the rest
of the group waits for the “report” of the
scout. Studies of animals in the wild
show that dolphins belong to a complex
society, one in which long-term partner-
ships of members of the same sex play an
204 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
FIGURE 9.28 A humpback whale (Megaptera novaeangliae) performing a full spinning breach.
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important role in sexual behavior, parental
care, and other aspects of daily life. Social
behavior in cetaceans may ultimately show
many parallels with social behavior in
large-brained mammals such as apes and
humans.
•
Marine mammals, particularly cetaceans, use
a rich variety of vocalizations and tactile and
visual signals to communicate with each other.
Play behavior and mutual assistance are ad-
ditional evidence of the complexity of their 
behavior.
•
The relationship between dolphins
and humans is a controversial one. Some
people swear of experiencing spiritual in-
spiration while swimming among dolphins
during the “dolphin encounters” offered by
some resort hotels. Dolphins trained for
military purposes by the former Soviet
navy are being used to treat children suf-
fering from behavioral disorders. Others
see this as outright exploitation of the cap-
tive animals. It has been suggested that
stress among captive dolphins reduces
their life span. Though exaggerations
abound, there are authenticated cases of
dolphins approaching human swimmers
who appeared to be in trouble. For more
than a century, fishers in southern Brazil
have established a unique partnership with
dolphins. The dolphins detect fish and de-
liver them to fishers waiting with nets.
Fishers have learned to interpret cues
given by the dolphins about the location
and abundance of fish. Generations of
dolphins have learned that a row of fishers
holding a net in shallow water means an
easy catch for themselves, even if it has to
be shared with funny-looking, two-legged
mammals.
One of the mysteries of the behavior
of whales and dolphins is the stranding,
or beaching, of individuals, sometimes
dozens, on beaches (Fig. 9.30). The ani-
mals refuse to move, and efforts to move
them into deeper water usually fail. Even
if they are pulled out to sea, they often
beach themselves again. The whales die
because their internal organs collapse
without the support of the water. Strand-
ing has been described in many species,
but some, such as pilot and sperm whales,
strand themselves more often than oth-
ers. It appears that whales become
stranded when they follow one or more
members of their group that have become
disoriented by a storm, illness, or injury.
This indicates the strong cohesiveness
and herd instinct of the group.
Migrations
Many pinnipeds and cetaceans make sea-
sonal migrations, often traveling thousands
of miles from feeding grounds to breeding
areas. Most toothed whales, on the other
hand, do not migrate at all, though they
may move about in search of food.
Chapter 9 Marine Reptiles, Birds, and Mammals 205
(a)
(b) (c)
FIGURE 9.29 Whale watchers may be rewarded with examples of the complex behavior of whales: (a) “spying” behavior in killer whales (Orcinus
orca) in the wild, (b) sperm whales (Physeter macrocephalus) surrounding an injured member of a pod, (c) two bottlenose dolphins (Tursiops truncatus)
carrying a stunned companion to the surface to breathe.
FIGURE 9.30 These pilot whales
(Globicephala melas) stranded themselves on a
beach in Cape Cod, Massachusetts. Only two
of the 55 stranded whales survived.
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The migrations of the great whales
are by far the most remarkable. Many
baleen whales congregate to feed during
the summer in the productive waters of
the polar regions of both hemispheres,
where huge concentrations of diatoms
and krill thrive in the long days. During
the winter they migrate to warmer waters
to breed. The seasons are reversed in the
Northern and Southern Hemispheres, so
when some humpback whales are winter-
ing in the Hawaiian Islands or the West
Indies, other humpbacks living in the
Southern Hemisphere are feeding around
Antarctica during the southern summer
(Fig. 9.31).
•
Most great whales migrate from winter breed-
ing areas in the tropics to summer feeding areas
in colder waters.
•
The migratory route of the gray whale
is the best known of any of the great
whales (Fig. 9.31). From the end of May
to late September the whales feed in shal-
low water in the northern Bering, Beau-
fort, and East Siberian seas. They begin
moving south in late September when ice
begins to form. By November they begin
crossing through the eastern Aleutian Is-
lands. They eat less while on the move,
burning off close to a quarter of their body
weight. The whales cover about 185 km
(115 mi) per day. They travel alone or in
small groups along the coast of the Gulf of
Alaska and down the western coast of
North America en route to the Baja Cali-
fornia Peninsula in Mexico (Fig. 9.22).
Migrating individuals often show spying
behavior, pushing their heads out of the
water. This raises the possibility that they
navigate by using memorized landmarks.
They reach Oregon around late November
or early December and San Francisco by
mid-December. Females generally migrate
earlier. By late February pregnant females
are the first to appear in shallow, quiet la-
goons in southern Baja California and the
206 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
Humpback whales
Summer
feeding 
Summer
feeding 
Winter mating
and calving
Winter mating
and calving
Gray whales
Arctic Ocean
North
America
South
America
Gulf of
California
Greenland Iceland
Europe
Africa
West
Indies
Asia
Mariana
Islands
Coral
Sea Fiji
Australia
Hawaiian
Islands
Equator Galápagos
Islands
Antarctica
Cape Verde
Islands
FIGURE 9.31 Migration routes of humpback (Megaptera novaeangliae) and gray whales (Eschrichtius robustus). Both species tend to migrate and
breed close to shore, where they were easily hunted. Both species are on the comeback. The gray whale was removed from the endangered species list in
1994. The western Pacific populationof grays that may still breed south of Korea also appears to be making a comeback. Gray whales used to live in the
North Atlantic until exterminated in the last century. Also see Fig. 9.22.
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southern mainland coast of the Gulf of
California. It is here that females give
birth and males mate with non-pregnant
females.
The northbound migration begins 
by March, after the birth of the 700- to
1,400-kg (1,500- to 3,000-lb) calves. Fe-
males mate every two years, and the first
to migrate north are the newly pregnant
females that did not give birth. They will
return 12 months later to give birth.
Mothers with calves leave last. On the way
north the whales tend to stay farther from
the coast and move slower, an average of
80 km (50 mi) per day, because of the
newborn calves and unfavorable currents.
The last whales leave the coast off Wash-
ington state by early May. They start
reaching their feeding areas by late May,
completing an amazing eight-month trip
of up to 18,000 km (11,200 mi), the
longest migration of any mammal.
There is still much to be learned about
the migrations of the gray whale and other
whales. It has been found, for instance,
that some isolated groups of gray whales
along the migratory route do not migrate
at all. This is the case in a group that re-
sides in the Queen Charlotte Islands off
the coast of British Columbia. Scientists
are using novel ways to investigate the mi-
gration of whales. Attaching small radio
transmitters to whales and tracking their
movements by satellite promises to un-
cover intriguing details. Gray whales are
known to avoid cities by moving away
from the coast. Females and young may
slow down their migration back to the
Arctic by taking shelter in kelp forests to
avoid killer whales. Analysis of the DNA
of humpback whale populations in the
Hawaiian Islands suggests that, as in the
green turtle, individuals always return to
the feeding grounds of their mothers. An-
other vexing question is how whales navi-
gate. It has been suggested that they use
the earth’s magnetic field, a possibility that
implies that they must carry some type of
internal compass to orient themselves.
Reproduction
The reproductive system of marine mam-
mals is similar to that of land mammals.
They have some unique adaptations to life
in the water, however. To keep the body
streamlined, male cetaceans and most
other marine mammals have an internal
penis and testes. The penis, which in blue
whales is over 3 m (10 ft) long, is kept rigid
by a bone. It is extruded just before copula-
tion through the genital slit, an opening
anterior to the anus (see Fig. 9.15).
Pinnipeds breed on land or ice, some
migrating long distances to isolated islands
to do so. In most species of seals each adult
male breeds with only one female. Cam-
corders attached to animals in the wild (see
“Eyes (and Ears) in the Ocean,” p. 11)
have shown that male harbor seals make
rumbling noises, quiver their necks, and
release a stream of air bubbles, perhaps a
display to attract females. In sea lions, fur
seals, and elephant seals, however, a male
breeds with many females. During the
breeding season the males of these species,
who are much bigger and heavier than fe-
males, come ashore and establish breeding
territories. They stop eating and defend
their territories by constant, violent fight-
ing. They herd harems of as many as 50
females onto their territories and keep
other males away (Fig. 9.32). Only the
strongest males can hold territories and
breed. The others gather into bachelor
groups and spend much of their time try-
ing to sneak into harems for a quick copu-
lation. Defending the harem is exhausting
work, and dominant males “burn out” after
a year or two, making way for newcomers.
It nevertheless pays off in the huge number
of offspring they leave compared with the
males that never reach dominance, even
though the subordinate males live longer!
Chapter 9 Marine Reptiles, Birds, and Mammals 207
FIGURE 9.32 (a) A male Steller sea lion (Eumetopias jubatus) guarding his harem on a rocky island off the coast of Alaska. Steller sea lions are the
largest of the eared seals; males may weigh nearly 900 kg (1 ton). (b) A harem of female California sea lions (Zalophus californianus) on Santa Barbara
Island, Southern California. The harem (center) is being guarded by a large, darker bull (top left). Large female elephant seals (Mirounga angustirostris)
rest near the harem, oblivious to the occasional fights between the bull and rival males around the harem.
(a) (b)
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Female pinnipeds give birth to their
pups on shore. They seem to be indiffer-
ent to the birth process but soon estab-
lish a close relationship with the pup
(Fig. 9.33). Because females continue to
go to sea to feed, they must learn to rec-
ognize their own pups out of all the oth-
ers by sound and smell. The pups
generally cannot swim at birth. They are
nursed for periods of four days to two
years, depending on the species. Most
pinnipeds have two pairs of mammary
glands that produce a fat-rich milk ideal
for the rapid development of the pup’s
blubber.
A female pinniped can become
pregnant only during a brief period after
ovulation, the release of an egg by her
ovaries. This occurs just days or weeks
after the birth of her pup. Females of
most species return to the breeding
grounds only once a year. By contrast,
gestation, the length of time it takes the
embryo to develop, is less than a year.
This difference would cause the pup to
be born too early, before the mother re-
turns to the breeding ground. To pre-
vent this, the newly formed embryo
stops developing and remains dormant
in the female’s womb, the uterus. After
a delay of as long as four months, the
embryo finally attaches to the inner wall
of the uterus and continues its normal
development. This phenomenon, known
as delayed implantation, allows pin-
nipeds to prolong the embryo’s develop-
ment so that the timing of birth
coincides with the female’s arrival at the
safety of shore.
•
Delayed implantation allows pinnipeds to time
the birth of pups with the arrival of pregnant
females in breeding areas.
•
Our knowledge of the reproductive
behavior of cetaceans in their natural en-
vironment is limited. We do know that
cetaceans are intensely sexual animals. Sex
play is an important component of the be-
havior of captive dolphins. Like humans,
they appear to use sex not only for procre-
ation, but for pleasure as well. Sexual be-
havior appears to have a role in the
establishment and maintenance of bonds
among all individuals, not just potential
mates. The sexes are typically segregated
within the pod, and males perform elabo-
rate courtship displays to catch the atten-
tion of potentially receptive females.
Fights among rival males are common,
but cooperation also occurs sometimes.
Gray whales are known to copulate with
the help of a third party, another male
that helps support the female (Fig. 9.34a).
Group matings have been observed in
humpback and white whales. Consider-
able touching and rubbing is known to
precede copulation (Fig. 9.34b). Actual
copulation lasts less than a minute but is
repeated frequently.
208 Part Two Life in the Marine Environment www.mhhe.com/marinebiology
FIGURE 9.33 A California sea lion
(Zalophus californianus) with nursing pup.
(a)
(b)
FIGURE 9.34 Mating behavior in great
whales. (a) Gray whales (Eschrichtius robustus)
often mate with the help of a third party,
another male that props the female against the
matingmale. Actual copulation is reported to
last for just 30 to 60 seconds. (b) Courtship in
humpback whales (Megaptera novaeangliae)
includes rolling, slapping of the flukes, and
pairs surfacing vertically face to face.
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Gestation lasts for 11 or 12 months
in most cetaceans. An exception is the
sperm whale, which has a gestation period
of 16 months. Development in most
species of large baleen whales is relatively
fast for a mammal of their size. It is syn-
chronized with the annual migration to
warm waters. It is remarkable that it takes
9 months for a 3-kg (7-lb) human baby to
develop, but a 2,700-kg (3-ton) blue
whale calf needs only about 11 months!
The calves of probably all cetaceans
are born tail-first (Fig. 9.35). This allows
them to remain attached to the placenta,
which provides oxygenated blood from
the mother, for as long as possible to 
prevent oxygen deprivation. The calf im-
mediately swims to the surface. In captive
dolphins, the mother or an attending 
female may help the calf to the surface.
Fat-rich milk is responsible for the rapid
growth of calves, particularly in the great
whales. They are born without their full
complement of blubber and must gain
weight before migrating with their moth-
ers to feeding grounds in polar waters. It
has been estimated that a typical blue
whale calf gains 90 kg (200 lb) in weight
and 4 cm (1.5 in) in length every day for
the first seven months of its life! The
mother’s milk is produced by two mam-
mary glands with nipples located on both
sides of the genital slit (see Fig. 9.15).
The milk is squirted into the calf’s mouth,
which allows the calf to drink underwater.
In at least some of the great whales, fe-
males do not seem to feed much while
they are nursing. The calves are not
weaned until they arrive at the feeding
grounds. In some species they continue to
nurse for more than a year after birth.
The relationship between mother
and calf during the nursing period is very
close. Frequent contact and vocalizations
are used in communication. Mother
whales are known to defend their calves
when there is danger. There is a report of
a female gray whale lifting her calf onto
her flipper to save it from the attacks of
killer whales. The bond between mother
and calf probably lasts for several years.
Captive young dolphins are known to re-
turn to their mothers for comfort in times
of danger or stress.
Cetaceans reach sexual maturity rela-
tively early, at age 5 to 10 in great whales.
Most females, however, give birth to only
a single calf—occasionally twins—every
two or three years. This low birthrate,
coupled with extensive hunting, may have
already sealed the fate of some of the
great whales.
Great whales have been estimated 
to live at least 30 to 40 years on average.
Humpbacks are known to live at least 
50 years, bowheads 150 years.
Chapter 9 Marine Reptiles, Birds, and Mammals 209
FIGURE 9.35 A Commerson’s dolphin (Cephalorhynchus commersoni) giving birth in
captivity. Not much is known about this dolphin, which is found only in southern South America.
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Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
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Companies, 2003
i n t e r a c t i v e e x p l o r a t i o n
Check out the Online Learning Center at www.mhhe.com/marinebiology and click on the
cover of Marine Biology for interactive versions of the following activities.
210
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Do-It-Yourself Summary
A fill-in-the-blank summary is available in the Online Learning
Center, which allows you to review and check your understanding
of this chapter’s subject material.
Key Terms
All key terms from this chapter can be viewed by term, or by defi-
nition, when studied as flashcards in the Online Learning Center.
Critical Thinking
1. Sea turtles have disappeared from many regions, and one way
of trying to save them is to reintroduce them to areas where
they have been wiped out. This is done by reburying eggs or
by releasing newborn baby turtles on beaches. Why are eggs
reburied or baby turtles released instead of fully grown
individuals?
2. Most seabirds are specialists that feed on particular types of 
fish and other prey. In some cases this may reduce the chances
of competing with other species of seabirds for limited
resources. Sometimes, however, we find two or more species 
of seabirds feeding on the same type of fish. What type of
mechanisms might have evolved to prevent direct competition?
3. Cetaceans give birth to few well-developed calves at well-
spaced intervals. They also feed and protect the calves for
long periods. This is in sharp contrast to most fishes, in
which many eggs are spawned and the parents spend no time
feeding and protecting the offspring. What do you think is
the best strategy? Has this effort paid off in the great whales?
For Further Reading
Some of the recommended readings listed below may be available
online. These are indicated by this symbol , and will contain
live links when you visit this page in the Online Learning Center.
General Interest
Chadwick, D. H., 1999. Listening to humpbacks. National
Geographic, vol. 196, no. 1, July, pp. 110–129. Marine
biologists study the migration patterns and social behavior of
humpback whales in the Pacific.
Chadwick, D. H., 2001. Pursuing the minke. National Geographic,
vol. 199, no. 4, April, pp. 58–71. The most abundant baleen
whale, minkes are increasingly being pursued by whalers.
Chadwick, D. H., 2001. Evolution of whales. National Geographic,
vol. 2000, no. 5, November, pp. 64–77. New discoveries have
helped us understand how cetaceans evolved from land-
dwelling mammals.
Geber, L. R., D. P. DeMaster and S. P. Roberts, 2000.
Measuring success in conservation. American Scientist,
vol. 88, no. 4, July–August, pp. 316–324. Populations of
some whales are on the increase while others are not.
Perhaps some of the species may not need help from
conservation efforts after all.
Hrynyshyn, J., 2000. The old man of the sea. New Scientist,
vol. 168, no. 2265, 18 November, pp. 44–46. Some of the
few surviving bowhead whales have been shown to be at least
200 years old.
Kemper, S., 1999. The ‘sea canary’ sings the blues.
Smithsonian, vol. 30, no. 8, November, pp. 86–96. PCBs and
other problems threaten endangered populations of beluga
whales.
Levy, S., 1999. What’s wrong with the right whale? New
Scientist, vol. 164, no. 2211, 6 November, pp. 38–42. 
Right whales get tangled up in fishing gear or collide 
with ships.
Martin, G., 1999. The great white’s ways. Discover, vol. 20,
no. 6, June, pp. 54–61. Video cameras attached to elephant
seals help scientists find out when and where great white
sharks attack.
McClintock, J., 2000. Baywatch. Discover, vol. 21, no. 3,
March, pp. 64–69. A long-running study of dolphins in the
wild reveals complex social relationships.
Motani, R., 2000. Ruler of the Jurassic seas. Scientific
American, vol. 283, no. 6, December, pp. 52–59. 
Ichthyosaurs, fish-like reptiles, ruled the seas for 155 million
years.
Nevitt, G., 1999. Foraging by seabirds on an olfactory landscape.
American Scientist, vol. 87, no. 1, January–February,
pp. 46–53. Response to particular odors helps petrels and
albatrosses find food in the ocean surface.
Pitman, R. L. and S. J. Chivers, 1998/1999. Terror in black and
white. Natural History, vol. 107, no. 9, December/January,
pp. 26–29. Killer whales attack a pod of sperm whales off the
coast of California.
Safina, C., 2001. Albatross wanderings. Audubon, vol. 103, no. 1,
January–February, pp. 70–77. Albatrosses travel thousands of
miles over the North Pacific to feed their chicks.Castro−Huber: Marine 
Biology, Fourth Edition
II. Life in the Marine 
Environment
9. Marine Reptiles, Birds, 
and Mammals
© The McGraw−Hill 
Companies, 2003
In Depth
Bowen, W. D., 1997. Role of marine mammals in aquatic
ecosystems. Marine Ecology Progress Series, vol. 158,
pp. 267–274.
Claphan, P. J., S. B. Young and R. L. Brownell, 1999. Baleen
whales: Conservation issues and the status of the most
endangered species. Mammal Review, vol. 29, pp. 35–60.
Pauly, D., A. W. Trites, E. Capuli and V. Christensen, 1998.
Diet composition and trophic levels of marine mammals.
ICES Journal of Marine Science, vol. 55, pp. 467–481.
Schreer, J. F., K. M. Kovacs and R. J. O’Hara Hines, 2001.
Comparative diving patterns of pinnipeds and seabirds.
Ecological Monographs, vol. 71, pp. 137–162.
Watt, J., D. B. Siniff and J. A. Estes, 2000. Interdecadal patterns
of population and dietary change in sea otters at Amchitka
Island, Alaska. Oecologia, vol. 124, pp. 289–298.
Whitehead, H., 1998. Cultural selection and genetic diversity in
matrilineal whales. Science, vol. 282, pp. 1708–1711.
See It in Motion
Video footage of the following animals and their behaviors can be
found for this chapter on the Online Learning Center:
• Egret eating (South Carolina)
• Humpback whale slapping pectoral fin (Alaska)
• Humpback whale spouting, diving (Alaska)
• Marine iguana (Galápagos Islands)
• Sea otter (Alaska)
• Sea lions (Gulf of California)
• Hawksbill turtle (Belize)
• Manatees (Florida)
Marine Biology on the Net
To further investigate the material discussed in this chapter, visit
the Online Learning Center and explore selected web links to re-
lated topics.
• Class Reptilia
• Marine turtles
• Order Crocodilia
• Suborder Serpentes
• Conservation issues concerning reptiles
• Class Aves
• Marine birds
• Conservation issues concerning birds
• Class Mammalia
• Marine mammals
• Resources from the sea
Quiz Yourself
Take the online quiz for this chapter to test your knowledge.
Chapter 9 Marine Reptiles, Birds, and Mammals 211